When restarted MacOs with reopen windows option I've got rubymine window minimized to just this small box. It's really annoying because it's difficult to find when other windows cover it.
JetBrain's Rubymine v2020.3.1 (Dec 2020) introduces compatibility with Apple Silicon (M1) Macs (and likely bug fixes) if you are using an M1 machine. If you are running the 2021.1 EAP3 (Feb 2021) build, that is essentially beta code and caveat emptor.
You are also now two macOS 11 updates behind and should update to macOS 11.2.1, which may improve your UI experience with Rubymine.
Failing the above, contact JetBrains.
Have you contacted the rubymine developers. Seems like a problem with their app as all your other windows appear normally.
MacOs Big Sur Version 11.1 (20C69) bug: automatically minimized window after restart
