This is an old question but it exactly describes what's happening to me.
Lenovo T470
Windows 10 Pro
Notebook is shared with one other user who is not viewing or editing
I have made a list of lines of text and gone back to flesh them out (my "headings")
If I press enter twice, everything south of the cursor is deleted, as well as several lines above where the cursor is.
The same behavior is exhibited whether the "heading" is in
with plain text
with underlined text
with bold text
is a list item or not
is a numbered item or not
