I also thought that perhaps the various adblockers and do-not-track-mes that I have on my laptop were the source, but nah. It also happens on the ole Pixel 5 phone.
It's not so bad now that I've realised I can just drop down to 'show all notifications', so at least that way I can still see messages.
I'm grateful that people looked into it, though. Thank you, MUSB admins.