You can play in Solo/Co-op and unlock items against AI, but Tier Mastery stops after a short time. Not sure exactly what it is.
Whenever there is a new 'vault' weapon, I go into Solo and get them unlocked playing the bots in a single match of Breakthru.
I don't know what happened either, but I was able to play for about an hour and unlocked about 20-25 tiers (on the battle pass). Usually (when I play against the AI) I can unlock no more than 2-3 tiers at the very most. I kept getting indications at the top of the screen telling me that I'd completed a mission from a previous week, and it shows that they've been completed despite my not having played against human opponents.
