I'm experiencing similar things:

• Posts made here not appearing on other federated instances
• Posts made by other federated instances disappearing or never appearing in communities here

I've checked with alt accounts created on lemm.ee and sh.itjust.works which are subscribed to the same communities.

@ruud@lemmy.world Is there a federation issue?

Edit: I think this is a problem with all communities on lemmy.ml. I'm new to the fediverse concept, but the issue seems to specifically be with them.