Playback problems using independent Voice Playback

I have this scenario:
In a piece for choir I have systems for sopran/alt and tenor/bass. In the Play tab I have chosen enable independent Voice Playback for both. Now I have created a VST Instrument with an instance of Omnisphere for each of th 4 voices, so 4 separate instances named accordingly (Omnisphere sopran and so on).
Problem: I want to create rehearsal tracks for each of the voices and I do it by soloing first the sopran + piano and create a mp3, BUT there are drop outs in the recordings! Sometime notes are not played back.
If I set the alt to Dorico Beep there is no problem. So my conclusion is, that somehow the two voices blocks each other even if one of them are muted!

What can be reason for this?

Presumably your piano is also a separate VST. Maybe try using a different VST like Halion or such for your voice part instead of Omnisphere and see what happens when you export an mp3. It might be an Omnisphere specific problem.

I don’t think it’s Omnisphere, because I use it all the time in Cubase and Logic and it has never being acting strange or anything. But you are of course right, that I should test other vst as well, to see if it is Dorico that is the problem.