Hey there! I believe this is just server load related. If you leave the page alone, it will eventually change to ✅ Joined. That has been my experience at least.
Just a wild guess, because I haven't looked at the code / implementation yet in GitHub, but It's very possible they use a message queue to store subscription requests for processing. That is what the observed behavior would lead me to believe at least. If that's not the case, the delayed sub would be complete server delay while the request is processing...