Strange behavior with 404 pages

I wasn't able to reproduce this issue with a clean install of Drupal 10.

Thanks slydevil,
This is occuring on a site so I reported it. I saw your reaction; tested with drupal 10. Bud I use version 9.
I installed a fresh Drupal 9, make a 404 page and set /admin/config/system/site-information point to the 404 page I just created.
The problem don't appear. So I think this has something to do with my specific setup and/or settings.
I investigate on that install.

Thank you for your quick response and effort,
MLZR