This kind of think implies to me that the file on your computer that records when you last signed in is not being updated. On Windows that sometimes happens when SketchUp isn’t installed with the proper permsissions. @colin can probably tell you where that file is on Mac. Perhaps deleting it and signing in again would replace the file and keep you signed in for 28 days.
But why it’s suddenly started I don’t know. I recently updated SU to the latest 23.1x release, but have not knowingly made any other OS or system changes in the last couple of days.
I’ll see if @colin can tell me what Mac file I need to try to delete.