Hit similar issue with other aspects.
I use encrypted notes on a very limited basis. When I bought a very high resolution laptop 2 years ago, I ran into issues with how EN displayed - as in very, very tiny. I fixed that issue but in my encrypted notes, the font sizes were all messed up - some items larger, in some cases letters in the same text string jumped 2-3 points in size...
Since I have always felt that formatting in EN sucks and have said so only to get some EN apologist to say that they are working on "other things". I am working on moving to MS OneNote... Enough sidebar.
My immediate issue was an attempt to fix the above font sizes by selecting text and using the font size drop down to change the font. My to my surprise it simply tossed me out of the note as you have described. First thought was that I was inaccurate in selecting the font size so I tried it several times to no avail. Then, to my horror, I saw that the selected text for re-sizing had vanished!!! Great.
I mucked around a bit - then removed the note encryption entirely - and then found that in my attempt to fix the note that EN had "duplicated" the entire note text within the note all with strike out - three times that I can see. I know this happened as when I was trying to resize fonts, I had directly typed in some new text (which was duplicated and struck out as well.
I cannot share this note as it has sensitive info in it.
This is unacceptable behavior in a widely used product. I will be amazed to get a helpful response. BTW: my EN versions are current.