Federation.
When you subscribe to a different instance, it can take time for the subscription to be sent to the server you're subscribing to.
Sometimes, that data never even makes it.
Honestly, "Subscribe Pending" doesn't change much, it will still show posts in your feed, and you can still make posts and comments to the community.
However, like the "subscribe pending" issue, you may end up with some comments showing up, and others never showing up, on the instance you're posting to.
I've had lots of troubles with the random nature of which comments of mine post to other instances. with beehaw and lemmy.blahaj it is seemingly at random if a comment will show up on their instance or not. When I go visit threads on beehaw or blahaj and not through lemmy.ml, sometimes comments show, sometimes they don't. It really is random and just whether or not the data made it through.
These are small servers run by volunteers and they don't have racks and racks of failover servers like reddit. The amount of data passing back and forth between instances is HUGE and it seems like a lot of that data is getting "lost" in the transit, resulting in things like missing comments or "subscribe pending."
These are part of the growing pains of a federated service.