Keep in mind that I've been an eBay user for more than 20 years and have feedback of more than 3000 (i.e. I'm an experienced eBay user).
Link https://www.ebay.com/sch/i.html?_from=R40&_nkw=centerpoint+bolts&_sacat=0&_sop=15 which probably won't help here...but here it is. Today the new auction is now showing, but not the auction I bid on (with exact same search). The auction I bid on still has 9 hours left. Both auctions still show on the phone. Both auctions are actually being done by the same seller with the newer one not ending for a few days (and with a lower price). Luckily with the phone results I actually found a buy it now listing (that wasn't showing on either computer) that is a better price than both of the current auctions. This situation has really turned me off to eBay since now I feel like I may be constantly paying more than I had needed to because of listings that were not being shown (for all I know this problem may not be temporary and could have been occurring for years). To me this is the most serious thing that could go wrong with eBay...not showing ALL of the results when you are expecting ALL of them to be shown.
And now to go through your possibilities:
Too soon: The auction I'm bidding on is actually ending in 9 hours (so this wasn't the case). I refrained from including the link in the question since once it is complete it won't be of any use for someone to try and figure this out (https://www.ebay.com/itm/134262943872). When I did the search it had already been on eBay for like 7 days (and my bid was about that many days ago). My research specifically had to do with comparisons so I could decide if I would bid higher or leave it alone (since I had been outbid at this point).
Sorting by "price plus shipping: lowest first": Not sure if this particular option is relevant to the "We removed some search results..." situation, but anytime I see that message I always click to see all the results. I don't care if I have to weed through extra irrelevant results, I've always believed it is up to me to decide if they are irrelevant or not (and modify my search terms if needed). It should never be up to eBay to decide this. But, yes my default sort order is this. Aside from that, when I tried it on the other browsers, the other computer and the phone, I made sure my sort order was identical for all (and again, making sure eBay wasn't removing any of the search results). Having it show up on the phone with all the same settings/options shows something is different (or broken).
Wrong Category: Unless I specifically want a particular category, I always make sure eBay searches the entire site (and yes, I made sure to check that...these things are just habit for me). I do the same with Amazon which is worse about trying to shove you into a particular category.
No Shipping: I always have the item location of default or worldwide selected unless I specifically want US only. Although the second computer (not having ever used for eBay) doesn't show my correct zip code, regardless eBay picked up on the IP address I'm using and showed a zip code that is around 70 miles from me (same for the phone as I noted all were using the same internet connection). Both the auction I was bidding on and the one I didn't even know existed (from same seller) ships to anywhere in the US. No conditions are specified in my search. No location limit, no price limit, no auction type, etc. And as mentioned, running the exact same search today now shows the auction I'm bidding on (both on my computer and the other computer), but still doesn't show the newer one with the currently lower price. To me this definitely shows
On Vacation: Although not the case here (and generally never the case for an auction vs buy it now), both of the listings that I at least know were not showing are auctions. Between me and you...that would be a terrible mistake on behalf of the seller if somehow the option to be on vacation removed an ongoing auction. I'm pretty sure it doesn't and in these cases you would just get a notation on the auction page that says shipping could be delayed because of this (I could be wrong since bad immoral buyers eventually made me stop sell anything on eBay anymore so I'm way more experienced as a buyer than a seller). I do know for sure that buy it now won't show in a search result when the seller is on vacation and if I have already save it to my shopping cart than when I click on my shopping cart it shows the message at the top as well as a notation next to the item/seller (this is actually occurring right now in my shopping cart). But, no matter what this isn't the case with these auctions/seller.
I live in Washington state and since I was able to bid on one of the auctions, have it show up today (the one I'm bidding on, not the newer one) and both show on the phone location is not an issue. Neither are sponsored listings and I do know that sometimes creates an out of order listing. I use 'Brave' browser as my default which is awesome for preventing tracking (created by the makers of Firefox). But, I do know this can create a problem hence why I mentioned I used other browsers. I first fall back to Firefox when I think Brave could be an issue...but I also know Firefox has some blocking features in it. Neither specifically has an ad blocker installed into it (Brave just defaults to blocking ads). As a final fallback I run Edge with no blocking what so ever. Both Brave and Firefox test was where I was signed into my account. When I tested the search on Edge I did not sign into my account. On the second computer I only tested 1 browser (but just now tested on a second browser with same result). I own my own tech company and therefore am running behind a server that blocks entire ad serving domains. But...this is irrelevant since my phone, using the same internet connection and NOT my data connection, shows both auctions. Also, if it matters, I was using the Brave browser on my phone. Didn't bother to test it on my phone with a different browser since I was getting more results than both desktop computers with just the Brave browser.
I really appreciate your thoroughness of possibilities since these are probably all the same ones I had thought of. I have read about other people having similar issues but didn't think much of it. It just sort of reached a point though with this when I truly felt I'm potentially being screwed by either something done on purpose or accidental (i.e. code error/break). Trying to report this to eBay is really just an impossibility so I thought I'd post it to the eBay community to first see if someone knows something I hadn't thought of...or maybe, some eBay employee reads this and actually tries to figure out if they do have a problem (this is assuming they are not doing it on purpose since if they are, they won't change it unless enough people complain about it).
