This has been irritating me too, and unfortunately, the only answers seem to be impractical workarounds.
Here's my amateur analysis of the problem (I'm on an iMac, btw, so not sure how this plays out on the app). What seems to be the issue is that eBay has very stupidly hardened the division between the "browse" and "search" functions, so now you end up on one "track" or the other and can't switch anymore.
You can see which track you're on by the url. For example, if you search "pants" from the home page, it'll take you to a url that starts with "ebay.com/sch/." However, if you navigate to Collectibles>Militaria>Korean War, the url will start with "ebay.com/b/Korean-Militaria-1950-1953." ("sch" for search, "b" for browse). At that point, if you type "pants" into the search bar, by default it routes you from the /b/ to /sch/ by running a new All Categories search that brings up all 5.2 million listings for pants as if you had just searched from the homepage. The drop-down menu will only allow you to pick within Collectibles from their cluttered list of 34 categories, so then you'd need to click on both Militaria and Korean War again.
This is objectively inefficient and a downgrade in functionality. What used to be a single click has now turned into at least three or four, and you now have to be very mindful of your location in the subcategories labyrinth whenever you just want to refine the search results with a keyword.
I don't have a real solution, but I do have another workaround that semi-works for me: replace the "/b/Insert-Category-Here" part with "/sch" in the url. You have to delete the "Insert-Category-Here" portion too, or else it won't work.
So for example, if I navigate to the Sewing Machines and Sergers section, it'll bring up the URL "https://www.ebay.com/b/Sewing-Machines-Sergers/3118/bn_7714181." If want to do a keyword search like a normal person instead of sifting through a thousand probably-irrelevant refinements, I delete "b/Sewing-Machines-Sergers" and replace it with "sch." So you should end up with "https://www.ebay.com/sch/3118/bn_7714181."
Thankfully, at that point, it'll keep you on the search track, so you only have to do it once.
Best I can do. Cheers, and let's hope this is temporary.
