Newlines added in textfields of html based richt text editor, with extension turned off for the site

**Describe the bug**
Newlines are added at the end of input in some "rich text" fields, even when site is excluded from checking "Harper is disabled on this page".

**To Reproduce**
Unfortunately noticed this on a site that is not easily accessible, because it's a commercial tool (Matrix Requirements Software)
Will update, if I ever see this on other sites.
What I can say about the input field in question, is that it is an html based richt text WYSIWYG editor and the added element is `

 

`. Opening the text editor is enough to trigger the behavior, I am not even making any changes consciously.

**Expected behavior**
When disabled for a site, Harper does not alter my inputs in any way.
When enabled, inputs are only altered on request

**Screenshots**
N/A

**Platform**
Firefox

**Additional context**
N/A

The report names no source file, test, or code entry point, and the original site is not readily accessible. Start by reproducing the behavior in Firefox with Harper disabled for a site and an HTML rich-text editor; done means opening the editor no longer inserts an empty paragraph such as

 

.

Written by the indexing model from the issue text.