The new HTML control CKEditor has little transparency on the line break handling.

In particular,

1) the user does not see the difference between Enter (resulting in html-paragraph) and Shift-Enter (resulting in html-break)

2) the customer cannot set a default behavior for Enter (this would be technically possible via CKEditor config.js, but is not possible in D365)

As the new HTML editor differs in the default line break handling (defaulting to html-paragraph when pressing Enter), as compared to the old HTML control (defaulting to html-div or html-break when pressing Enter), this would ensure the behavior of the line break stays similar between old and new HTML control.