I feel your pain ....
The type of problem you are posting about (missing buttons, upload failures, etc.) has been one that has come up multiple times over the last couple (few) years, and there are other threads that have discussed it before, I myself started a thread back in 2017 discussing uploader issues (
viewtopic.php?f=4&t=310), and I've encountered the same type of issue multiple times over the years.
When the issue does rare its ugly head, some people seem to have the issue while others don't, like now, I do not have any issue uploading side-by-side pairs or left/right individual files with my account, I just tested both types and successfully uploaded a few files to my stream. The description was fine, the title was fine, and even the tags all uploaded with the images perfectly.
I am running Windows 10 64-bit with Firefox 64.0.2 at the moment when I tested uploads, but in the past (as my other thread disclosed) when Firefox broke with Phereo uploads I could sometimes use Microsoft's Edge or Explorer 11 browsers to successfully upload images without any issues at all.
So sometimes it seems to be specifically related to which OS and/or which browser you are using, and by changing browsers, refreshing browser cache clearing all site data some people (like myself at times) have been able to once again get the uploader to work . . . other times no-one has figured out what was wrong nor what steps to specifically take to immediately fix the issue . . . but eventually things just seem to go back to normal and working for everyone again, and everyone stops talking about it, and we all go back to uploading our images perfectly fine again like it was never there at all.
Unfortunately, the majority of the Phereo team more or less ceased most (all) support and operations years ago, though occasionally when things seemed to break fully, someone had come along and "fixed" things. There are a very limited number of users here that even post or follow the forum here often enough which might be able to possibly offer help in a timely manner, if they could help at all, and the admin and mods have basically disappeared and rarely (most of them never) come here and help anymore, which has also been discussed in other threads too, it's a sad state, but just something you have to get used to dealing with I'm afraid if you decide to continue to use Phereo for your 3D images, sorry.
~ Craig ~