Click on unread button n all unread msgs will be changed to read(this was the method I learned to get rid of persistent notifs-even when I clicked the post the noted remained unread until I learned this method).
This is similar/reminds me of email amnesty(when u delete emails without reading) only in this case u can go back n scroll down later, unless ur unread notifs go over 5 pages I think. Then old notifs if u dint read em disappear without any known way of retrieval.
I don't rmbr max notifs iv had but ard 10~15 when I was busy at/with/after work earlier this Yr, srry if anybody tagged me n I nvr replied-too busy/no time~mental energy to read/reply/do dailies during those mths.