This is a long-standing bug and the following issue explains it pretty well: [Bug]: Components steal X11 focus when clicked · Issue #1171 · juce-framework/JUCE · GitHub . Essentially, when you mouse-interact with a plugin editor, the editor would steal all keyboard events. And it would be quite annoying, e.g., if you want to use space to pause/continue playback.
I have created the following PR to address the issue. I have only tested on my local VM though 
I’m on kubuntu 26.04 + reaper and been having the same problem with all juce plugins. Not just key down but hovering onto a gui (while holding down a note - on a virtual keyboard, then letting go) will cause the note to be held until pressed again
Based on a user report, I am currently less confident about the fix, as it is described as working randomly. If the fix is beyond several missing flags, it might be beyond my capability.
It would be great if JUCE team could look into this
Meanwhile, if any Linux users/developers would try a plugin built with the fix, here you are: Release ZLEqualizer · ZL-Audio/ZLNightlyRelease · GitHub
