same problem is occuring to us right now. Just started today. Multiple teams, assignment tab is not there and says disabled. if you look in teams admin teams apps -> manage apps , the "assignment" app is "available to" everyone. I have users requesting access to it, but i cant give them access becuase they already have it....
Must be a microsoft bug. There is some chatter around, but i havent seen anything on /r/k-12 which i cant actually post to...