The effort is much appreciated!
Maybe the key to this problem is what I wrote before:
"I just noticed that when I reopened REAPER which opens up the last saved project(s) in one tab or multiple tabs - then there is that dissonance in the track(s). But when I close those last saved projects and I reopen the other version of my track, which previously sounded dissonant with that bug - it surprisingly sounded fine!"
And I continue from that: When I again close the REAPER, and reopen REAPER, then the track becomes again horrifyingly dissonant. (Back to square one.)
So - I have noticed that:
1) When opening REAPER again, the opening screen of REAPER does not treat the to-be-loaded/last-saved project (which has a Zenology Pro track) properly with 44.1kHz sample rate.
2) and only when one opens the project from within the REAPER: File -> Open Project... OR Recent Projects - only then REAPER treats the newly opened project (which has a Zenology Pro track) with 44.1kHz sample rate.
3) And when I close REAPER again after the phase 2, then reopen REAPER, then we are back to phase 1, and the 44.1kHz sample rate does not work with Zenology Pro.
So - there must be some fundamental difference when opening the project at start-up and when opening the project from the navigation bar. What that is, in terms of the computer code? I leave it developers.
I hope this helps to clarify things for the developers.