Seems like the Undo function has trouble understanding just how much to actually Undo. But it seem inconsistent in behavior
For example…
In the editor, type two words then return, then type a few more words then return, then a few ore words. Then Undo. ALL words are gone.
We’re relying on the macOS standard text view behavior here. I’m not sure what its calculations are based on, I assume it may have something to do with time and/or significant application events.
If you fire up TextEdit you should be seeing the same behavior. (though you may need to modify the text system preferences to disable the substitutions, as that seems to create undo points).