Unread messages / notifications generated on some teams for people that joined years ago

Been having a problem for a limited set of users and need some help figuring out what is going on here and how to resolve it.

(Unfortunately this is very disruptive to their workflow because they keep thinking there are messages or new things happening that are not happening and it’s creating a challenge for them to stay up-to-date and know when things are and are not important, etc. and thus it is causing them to deprioritize their contextual understanding of new and important messages.)

They are getting unread messages flags on some teams and channels for users that joined channels a very long time ago and that were already seen, a very long time ago. An example is:

It should be noted that this is set:

Any idea as to how this can be happening?