Covers are, unfortunately, one of the more complicated bits of FFF.
FYI, I use logger.debug() instead of print() and run in debug mode--I've had print()s not show up, especially in BG jobs.
Also, the cover image for
https://www.fimfiction.net/story/501587/ is relatively small (335px × 150px), do you use a
cover_min_size setting? (Seems unlikely that would be the case for
all your updates, tho.)