Strange calendar picker behavior on Mac + Mouse


I am on am MacBook Pro / Catalina. I have some issues selecting dates or date ranges in the calendar pickers when I try to do this with an external mouse (Mighty Mouse). It works perfect when I use the trackpad, but with a mouse click it doesn’t work properly. Most of the clicks do not work - no response at all. It’s like the event of a mouse click is triggered different than a tab/click on a trackpad. On mobile / touch devices it is also working like expected. Any ideas?