I am assuming this is a bug, since this seems like a big oversight. I saw a patron (of another creator) report their experience when changing their tiers. To quote them:
I switched to a lower tier bc I was broke, and they immediately took away almost all of the patreon bonuses and they would charge you again in order to go back even though it already took my money for that month.
If the patron already paid for the higher tier for that month, then switches to a lower tier, I don’t think it should take away the rewards they paid for.
I would encourage this patron to write to the Patreon support team what happened and how it was different than what they expected to happen. This will help the team understand what they’re experiencing and be able to look at both the patron’s account and the creator’s account to identify what’s going on, if it’s a bug, or if it’s a creator setting etc.
If it’s not a bug, they’ll work with the patron to explain and if possible, fix, what happened here. Without more details (and account info) it’s hard to tell what exactly is going on here.