Mine has done this for a few seconds on the communities I have tried joining, but then resolves to Joined. It may just be the system struggling to keep up with the influx of people. Myself included.

I have three "!subscribe pending"s in my list after a session of joining a few days back. For me, they are showing up in my sidebar and I can post to them as usual, it seems. It is just that they don’t change from ‘pending’.