Haven't been selling for a while, but had this same problem when listing back in September and October. It was extremely frustrating, as you're saying. The only way I was able to get more than one uploaded was to upload each one individually. Took forever, but it worked. Can't remember whether I had more success in desktop or mobile site. I had more success doing it from my phone, as well, for some unknown reason.
It's most likely a temporary glitch and will be fixed soon, hopefully. Also, I saved for later and reopened after every upload after I lost my work a few times. 😮 You may want to protect yourself by doing that, too, although it makes you want to tear your hair out and scream. 
