Could be related to the auto paragraph funcitonality of CKEditor, which is used by default and can be disabled with this !deprecated setting:
"autoParagraph = false"
This can be easily disabled using the ckeditor_config module, see https://www.drupal.org/project/drupal/issues/3131055 for more information.
The whole paragraph wrapping in CKEditor ...
I checked your HTML code thoroughly. The culprit is data-align="center" used in the img tags. To fix the problem, we have two options:
Option 1. (Recommended) If you remove it(data-align="center") from your code, your problem will be resolved, ie; the p tag won't be removed automatically when Source button is clicked.
Option 2. You can ...
I can't figure out how to "connect" an editor format to a field and
make it selectable for the one who adds a field to a content type.
The text formats are shown in order here:
The format at the top of the list is the default format. To change the order of the formats, drag the arrow icon on the leftmost side and ...
Issue solved by server admin. It was about ModSecurity Apache2 settings. POSTs were rejected because they triggered new ModSecurity XSS alerts. So server admin excluded some rules. He found them in Apache2 error log and exluced them in Apache 2 conf file.