Ok, not a big problem 
I’m trying to find out what’s wrong with rncbc plugins (GitHub - rncbc/drumkv1: drumkv1 - an old-school drum-kit sampler for example)
It works in Ardour, Jalv, Carla, Qtractor and Reaper. Qtractor and Carla both are Qt-based, so let assume they share some Qt stuff as you mentioned here `juce7` technical preview branch - #135 by reuk
Ardour and Jalv uses libsuil I guess: LV2 / suil · GitLab (at least, they linked with that).
And Reaper is a pure X11 app where drumkv1 works. It loses focus after you select sample so values cannot be set with keyboard, but this is a focus/window stacking problem, otherwise it works fine.
With JUCE it has focus and receives events from mouse/keyboard but nothing happens after. To check that, you can place a ~/.config/QtProject/qtlogging.ini file and switch Qt logging on:
[Rules]
*.debug=true
Then run APH in the terminal and watch the output. Every click is received, then nothing. The same check can be done in the other hosts (run Reaper in terminal and add this plugin) to compare plugin’s action.
My guess (very primitive) that problem is between Qt and JUCE, something prevents to execute threads when Qt apps was executed “inside” JUCE. I’d like to trace that more deeply but not sure how to.