The uploading functionality—either a file or multiple files—is broken. It's stuck on "Waiting to upload" forever. And files are not guaranteed to be uploaded.
Seriously, don't people at Proton do testing the Android app? I have to switch to use Firefox for using the web app. It's inconvenient, but that's the only way that uploading to Proton Drive in mobile works.
Did you try reinstalling? There were not much reported issues of this kind AFAIK, but you could always contact support here for specific issues: https://proton.me/support/contact (same as you posted).
It happened to me some times, but then it went back to normal.