mac mail imap suddenly stopped working

I have a macbook pro 13" (2018 touch bar model) running OS 10.14.6 Mojave. It seems to have stopped updating mac mail via imap. I have a few other macs all connected via mac mail to the same IMAP based account and usually if i read something on the laptop the other mac mail accounts will show the email as read.

They suddenly stopped doing that yesterday (i changed no settings obviously).

I assume its that my OS or something has messed up on with the version of mac mail im running.. Seems i cant update it without updating the whole OS which is now Monterey I believe?

Would that be the fix..? I hate updating OS when its all working fine, but now it isnt, so if i have to i will..?