Read receipts always move the wrong receipt avatar on read in a group room

[Half-Shot]

This has been a thing for ages, but it's finally annoyed me enough to create an issue (I did look around, couldn't find anything matching).

To reproduce:

• Open a room with at least 2 other people
• Talk in the room, ideally mentioning one of them.
• Watch as one of the avatars moves onto your message, to reflect that they have read it.
• BUT WAIT, that wasn't the person you mentioned. An eager reader...or maybe.
• Swap to another room and then back, and notice that the avatars are now correct.

So in summary, this is mildly annoying to the point where I no longer use the avatars as reference but assume that if A avatar moved, it was probably the person I actually talked to.

[turt2live]

Same cause as #9769