Just a heads-up here. I'm running KDE's Linux with KDE Plasma 6.8 (yes, 6.7 was just released today but I'm running the Dev 6.8) and the flatpack version of Joplin (3.6.14) that gets installed by the Discover app comes up with no menu bar. The appimage (3.6.14) from the Joplin website runs normally.
I updated to Plasma 6.7 yesterday (Fedora Kinoite) and the flatpak version of Joplin as well. I have issues with the vim keybindings. It is as if I am always in insert mode, with the Enter key not working.
I was having problems with the keyboard as well. There was no key repeat but I managed to trace that down to somehow the virtual keyboard got turned on. When I disabled that the real keyboard went back to normal.
Thanks for the trick @HarSel. This indeed solved my issue. Btw. it seems that the menu bar issue goes back to at least Plasma 6.6: