Previously, it was stated that only Kindles running firmware 5.15.1+ read covers from PDOC files:
https://www.mobileread.com/forums/sh...78&postcount=2
However, looks like something has recently changed and now 6th gen Kindles running firmware 5.12.2.2 now reads PDOC covers too. Maybe it goes back even further.
Source:
https://github.com/ciromattia/kcc/issues/1164