Steps to reproduce the issue

Based on the 'The 404 That Could: Why Your Joomla Site Deserves a Brilliant Error Page' article, I've tried to customize a 404 page.
However, I noticed some strange behavior in a certain situation that I can't seem to resolve.
I created a custom module as described in the guide, where I also inserted a custom text and an image.
Now, the behavior is as follows:
If I type http://yourdomain.com/thispagedoesnotexist -> the custom page appears correctly and the image is also visible.
If I type http://yourdomain.com/category/thispagedoesnotexist -> the custom error page does not load the image.
The image src is relative and the link ‘Back to homepage’ does not point to the homepage.

Any idea how to solve this issue?

Expected result

Actual result

System information (as much as possible)

Joomla 6.0.1
PHP 8.3.29
Child template from Cassiopeia

Additional comments