I have experienced this when lemmy.ml was having issues. my instance was aware of the community existence but because whatever was happening with lemmy.ml when i tried to access the community (for me was startrek) it couldnt fetch any post data.
my instance then took a few hours to retry (best guess as i have no clue how the protocol/process works) and now all is visible.
I also noticed that that this was because i was the first person on instance to attempt to access the !startrek@lemmy.ml.
(edit for formating)
Well.. we can thank Turkish people for their ever so wise voting decisions.... /s