This is a small bug that I noticed while working on #8956.
We have a for-loop which attempts to strip all presence changes for each user except for the final one, as we don't really care about older presence:
9e19c6aab4/synapse/handlers/presence.py (L368-L371)
`new_states_dict` stores this stripped copy of latest presence state for each user, before it is... put into a new variable `new_state`, which is just overridden by the subsequent for loop.
I believe this was instead meant to override `new_states`. Without doing so, it effectively meant:
1. The for loop had no effect.
2. We were still processing old presence state for users.
