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It happens when you subscribe to a community on another instance. I don't know any of the details but I'm assuming that subscribing doesn't require approval.
I think instances communicate by bundling requests into larger batches sent every minute or so to save on network overhead. As such, it may take a while for low-priority interactions to get through.
Pretty sure it's just bugged. I subscribed to !pathofexile@pathofexile-discuss.com over a month ago and it still shows "pending".
There is no approval process. It can be a somewhat expensive operation of no one on your instance has subscribed to that community yet. That's why it takes time.
i've never heard of a functionality like that.
maybe the instance where the community is located is a bit overloaded? or maybe is a front end thing