To answer your question:
I am currently using a bare RPi5 powered by an official 27W Raspberry Pi PSU. No case.
The USBs being tested with are rather small (16 GB). But while FAT32 does never lead to any problems, when formatted with NTFS this clearly breaks LibreELEC.
However, when testing these USBs separately on either Windows machines or Raspberry OS (using RPi4 in this case) the NTFS devices gives no problems at all.
So clearly LibreELEC is less "forgiving" about my NTFS USB devices than the standard Raspberry OS.
Thanks for bearing with me and answering my questions so far. I will attempt to do some more testing and see if I can come up with a log file. And I will look more into whether power might be a culprit.