Reporting back for future generations, I think I've found the issue. I was retracing my steps and found this: When installing postmarketOS there is a device specific instructionto run "fastboot oem uart enable" command. So I tried to run "fastboot oem uart disable" and then flash UT. So far working like a charm. Did a switch off, did a restart and all works as I'd expect it - also with Libertine set up.

@ernest Many thanks for your help! Sorry, I should have searched for a hardware issue before posting here.

Thanks, @Luksus.