Interactive plots not showing

When displaying matplotlib plots with jupyter (console / notebook / lab) inline, everything is working fine. When switching to an external window, e.g. with matplotlib or matplotlib TkAgg, things are working also with no problems when I use Python 3.8.x versions. But with Python 3.9.x, the plots get briefly started, and then immediately disappear. I can arrest this by inserting a keyboard input:

I am running Windows 10, and have installed Python with WinPython distributions. I tried it on different computers, and could reproduce the problem there.

Surprisingly, I cannot find many related posts on the web. And updating to the latest jupyter packages makes no difference.

Does anybody have an idea what could cause this?