Radium

Maybe it's related to Zyn 3.0.5?

Probably not, but maybe. I still think active sensing is the reason though.
Is there any way to turn off active sensing on your MIDI keyboard?

Not sure if I get it right: active sensing means things like aftertouch?

Active sensing is a realtime MIDI message (0xfe) that MIDI keyboards sends regularly to tell that they are alive. If a synthesizer suddenly stops receiving active sensing messages, they should stop playing to prevent hanging notes. It's exactly what seems to be happening for you. On my synthesizer, there is an option to turn it off.
Here's an LV2 plugin to remove active sensing messages, but I don't now how you would use it with radium: https://x42.github.io/midifilter.lv2/ht ... nsing.html

Got it!
I searched a possibility to turn off active sensing for my keyboard inside the config-software, but didn't find one.
Just to try it, I disabled polyphonic aftertouch. This makes the problem go away. Seems like this was the cause of the problem.

Strange. I guess the other hosts you've used filters out polyphonic aftertouch messages. It may seem like the same problem as this one: https://sourceforge.net/p/zynaddsubfx/m ... /29359693/ The solution seems to be to toggle something called "SynthNote.legato.silent" in zynaddsubfx.

Okay thanks for the tip. What counts is, that I know now how to get around it.