I'm using Pop_OS 22.04 with Gnome DE. Underlying it should be very similar to Ubuntu.
So I use JRiver on Linux every day, and have not seen 2-4 at all. I very occasionally see 1), but it's intermittent and I can't reproduce it on demand so I haven't reported it. The other issues are probably something special about your setup.
Some thoughts:
Did you backup and restore a media library from a Windows version of JRiver or does your JRiver for Linux machine have a completely new library?
I ask because number 3) on your list sounds like a very specific failure mode from bringing across a library from one platform to another that I've seen before. If you're using a library that started on Windows, check the "platform.jmd" file (probably at ~/.jriver/Media\ Center 32/Library/platform.jmd), and see what the file has in it; if it doesn't say "[Platform]=Linux" that's going to cause a lot of problems on Linux. It will specifically cause lots of slash versus backslash issues, and can also mess up other file access stuff. That may not be your issue, but it's the main thing I've seen cause weird slash/backslash issues.
Alternatively, is the machine in question a client to a JRiver server somewhere? If so, 4 is expected behavior (clients can't ever change cover art), but that behavior is not normal on Linux other than in a client/server context.