adding div element attribute to mystyles.js can not be used (vanishes after save)

[Smirftsch]

I've got some odd problem adding additional styles to mystyles.js. After reading some and following the article here: https://github.com/ryancramerdesign/ProcessWire/blob/dev/wire/modules/Inputfield/InputfieldCKEditor/README.md#custom-editor-js-styles-set

I was able to add a custom style. So far so good ?

It is displayed in the Styles menu and can be selected there. Now the odd thing starts, if I add a custom style like this:
 

...

 { name: 'Box Top', element: 'span',attributes: { 'class': 'box_top' } },
 { name: 'Box Bottom', element: 'span', attributes: { 'class': 'box_bottom' } }

 

it works and can be selected just fine.

However, if I add this:

 { name: 'Box Top', element: 'div',attributes: { 'class': 'box_top' } },
 { name: 'Box Bottom', element: 'div', attributes: { 'class': 'box_bottom' } }

It is also displayed, can be selected - BUT - once the edit is saved, it is gone, won't be displayed in the page and won't be shown anymore in the editor as selected style, it goes back to "normal".

 

Can anyone give me a hint what I missed?

[Smirftsch]

To answer the question myself, in order someone else may find this useful: It was "convert div tags to paragraph tags" blowing up on me. Thanks nevertheless ?