Hi Florian,
we can reproduce a problem with the formatting when switching to the "new look" in D365.
It seems that the CSS within the HTML isnt used at all. We already updated it to the latest release.
"New look" turned off:

"New look" tuned on: (it also seems that another Editor is in use then, but i am not sure about that)

Thank you to have look into it in advance!
Marcus