Damn!
The last time I checked my real time configuration was around the end of 2019. At these days I was still using the stand alone versions of software synthesizers. I wasn't familiar with the use of their plugin versions or mainly thought, plugins are only for stuff like Equalizers, Compressors, Reverbs and stuff.
Since the last time I checked my real time configuration I switched the kernel used two times without checking again.
Started from 5.0.21-rt15 which was the real time kernel taken from the latest Puppy Studio aka Studio 1337 by Iowt3ch.
I switched to 5.4.5-rt3 and then to 5.11.4-rt11 which were created by another guy.
Right now, a few minutes ago (I changed some stuff in my OS and rebuilt it) I checked real time configuration again.
It returned, Kernel with real time preempt NOT FOUND!
Both of those kernels I used at last have been announced and published as being real time kernels.
I switched back to the kernel 5.0.21-rt15, rebooted and checked real time configuration again.
It returned, Kernel with real time preempt FOUND!
So, now I'm looking forward to the upcoming results of Xruns within my next compositions.
My conclusion of this: never ever trust any Guys -no matter if Linux or Windows- without checking the facts by yourself.