I hate it, but... goto the chat channel and click in it.
I had someone on the team here send one, I was already in the chat and it still showed up in Activity. I just click in the message area and it goes away.
I still haven't figured out the logic behind dismissing notifications. Sometimes they just get stuck forever.
But the worst bug is that from time to time the freaking arrow keys stop working in the edit box! Last time it happened to me it lasted weeks and survived reboots. I can find references to this issue from 2019.
How can such basic functionality be broken I have no idea.
I hate it, but... goto the chat channel and click in it.
I had someone on the team here send one, I was already in the chat and it still showed up in Activity. I just click in the message area and it goes away.
I hate Teams. So much.