[4.0] Impossible to save an Email Template in a HTML Format

[blueforce]

Steps to reproduce the issue

1. Go to System > Email Templates > Options
2. Set the Mailformat to HTML and save and close
3. Create Mail Template for eg. German
4. Switch HTML-Body to «yes» and insert a Text, an image and various formats (H1, H2) usw.
   I used the TinyMCE as default Editor

My Example looks like this

5. Save the content

Expected result

The content is saved

Actual result

The content is not saved and the Button from the HTML-Body is set back to «No»

System information (as much as possible)

PHP 7.2.x
Joomla 4.0 B1 dev Version Nightly Build from 26.03.
Nginx Webserver (localhost) and reproduced with joomla.com online Version

Additional comments

[Bakual]

@Hackwar Any idea?

[ReLater]

confirmed

[richard67]

Please test Pull Request #29229 . It should solve this issue.