i think it might be because you are from a different instance... it may just be taking a while or there may be some glitches.. just a guess. I had similar problems when trying to subscribe to something on an instance other than waveform... and its still hung-up.

lemmy.ml has been under a lot of stress and has just moved to a new server

It's probably that. Or a bug or config setup :)

We’ve been having issues with federation and I have been unable to figure out why. We need help from the lemmy team or someone else to find it.

I seem to have the same issue with any lemmy.ml communities I sub to. I am pulling data though. Not sure why it is stuck on pending.

I noticed that at least a few of my "Subscribe Pendng" communities are subscribed - I can see the posts in my Subscribed list. Unsub + resub cleared one or two of them as per this closed issue on GitHub;

https://github.com/LemmyNet/lemmy/issues/2685