Several AppImage issues (broken sound & menus)

I'm experiencing some strange, inconsistent, but recurring issues when using AppImages on Manjaro Linux (Xfce), and was wondering if others have similar issues, before testing multiple versions & OSs and filing separate issue reports. I usually use the nightly build and these occur in several versions, but as far as I remember, I also got these issues on stable AppImages ; but not on installs.

1.) Sometimes playback doesn't work - often this is foreshadowed by pops on startup, rarely by continuous shrieking noise until closed. The cursor follows along through the score, but no sound. Other apps can still play back on the same device, but their sound is distorted, unless I turn down Musescore's volume in the Pulseaudio mixer. This makes me suspect that Musescore/fluidsynth is perhaps giving out a constant non-zero DC signal? Anyway, sometimes closing & starting up the same version again fixes it. Seems random.
2.) Moving the window to a second monitor breaks dropdown menus (File etc., the workspace selector, right-click...) Moved back onto the laptop screen, everything works again. (This could very well be a bug in Xfce...)
3.) Often I can't quit using the menu or the top-right X-button, and have to resort to killing the process.

I'd be very interested if anyone has seen something similar, especially if they use a non-Xfce desktop environment on Linux. Also, advice on efficiently troubleshooting this is appreciated. <3