Some avatars missing from Messages on Mac

Recently many (but not all) of the avatars of people in my Messages app disappeared (see image below). As far as I can tell, this is only on my Macs. I have been living with this for over a month thinking maybe this issue would resolve itself, but it hasn't. Last week I got a new Mac Studio. I thought maybe a fresh start would resolve it, but I was wrong. The issue persists on a new machine.

One thing I have tried is manually updating the poster image and avatar on my iPhone for the people whose avatars are missing, and that fixed it for a little while, but then they all reverted.

UPDATE: The Mac is a 2019 MacBook Pro 16" on MacOS 14.3. The post incorrectly tagged my computer as a MacBook Pro 13" on MacOS 10.15.