Thanks for the Tip.
I tested the System with a classical Ubuntu (Ubuntu 18.04.4 LTS, Kernel Linux 5.3.0-53-generic) and in roughly 2800 bootcycles that problem never occurred. The clock drifted a bit (to be expected), but nothing more.
Checking the BIOS version is a bit more difficult at the moment, since I’m mostly not on side, but honestly, since it was fine with the normal Ubuntu, I would find it strange if there is an underlying BIOS problem.
Is there a difference in how Core deals with time management, RTC, ect. compared to the normal Ubuntu?