Hey,

Out of respect for the user, I don't want to want to link the actual case; it's not about them but the underlying mechanics.

A post on a community on beehaw.org doesn't show the comments of lemmy.dbzer0.com user when I visit it via piefed.social.
lemmy.dbzer0.com is a trusted on piefed.social. Even if I logout, I can't see the user's comment on piefed, so I don't think it's because I am blocked by them. Does this mean the user is blocked by PieFed.social, or how does that work?

Thanks!

Yes, that could well be it!

Ok, thanks :)

If you go to https://piefed.social/modlog and search for their name then there might be some history about them that tells you why. Or try going to their instance and to the modlog on there and search there as well.