Dear maintainer I just installed linux-image-5.10.0-21-amd64 (5.10.162-1) and tested again.
The problem still exists.The bug can be reproduced by constantly keeping the connection rate between the webserver and backend at around 15/sec. After some time (around 5 hours in my last test) the frontend starts reusing ephemeral ports even though it is configured not to.
Best Markus
