This has been an issue since the update to the Canvas discussion forum. If I would have to guess, they've changed the code for the text boxes everywhere in the LMS.
The additional line breaks are a result of excessive SPAN tags in the HTML. You can see this if you go into HTML code view of the text box.
Additional SPAN tags are inserted when copy/pasting content from any source, a Word document, the Canvas page if you (like me) tend to paste a snippet of a student's post into your reply, etc...
I observed this issue when using Firefox, so I tried Edge, but the problem did not manifest. I have not tested other browsers, because honestly that's Instructure's job!
You can eliminate all the extra SPAN tags and additional junk code by hand in HTML code view, or use Edge. Instructure really should test new features on a variety of browsers and under a variety of use cases before pushing a software update.