Can't mark threads here "seen"

At this AskLibre site the envelope after my name is red. Clicking on it leads to my Profile / Notifications page, where are the responses to the threads I have participated in. There are controls there to Select All and to Mark "Seen". the Select All control works, but in four browsers - three chromium-based and Pale Moon - the Mark Seen control does not.

See this thread -

Thank you, Peter. That is exactly the problem.