That is because your image has float:left but it is too big to be floated (4000px width!!).

I changed the image to 1920px, and it's the same problem.

1920px is still to big to be floated, you have only 986px space in the content area

All my images are in 1920px and it works. There is only this article with the orange warning box.
If I start the article with:

everything looks fine to me and I have zero idea what @drmenzelit is talking about. The image is scaled so the native size is irrelevant to the float

I have doubled the article so you can see the difference.
https://joomla-4.fr/informations/avertissements/comment-supprimer-lavertissement-php-de-joomla-2

Ok I can see it now and what is happening is perfectly correct for the code and css on the site (even if it is not what you wanted or expected to happen)

This isnt the place to go into great depth on the issue as its not really a core problem. But you can fix this in two different ways.

1. Dont put a float on the image
2. Remove position: relative on the alert

Note that you are misusing the alert anyway and breaking accessibility.

I added some introductory text before the warning box. Thanks a lot you two.

Yes, sorry, I was not clear enough telling you that removing float:left from the image would solve the problem. Float is ok when the image is small enough to share the place with the text.
I think you can now this issue.

@Magnytu2 it seems that this is not a bug or issue of joomla. Can we close this?

Closing as not a core issue. Thanks.