According to check issue #33599 I found new issue, that describes in title.

Steps to reproduce the issue

Rename your testing site to my example;
For additional information check #33599 in following section

Expected result

Sitename does not go beyond the block

Or it will be better, I think, make another PR to move sitename in 403 error template to right corner (highlighted red colour).

Actual result

System information (as much as possible)

Latest nightly Joomla 4.0.0-beta8-dev, Firefox 89.0b8.

Additional comments