Upgrading mintty fixes this bug.

I was experiencing this when using a very old Cygwin install (pre-2015) on Windows and was using the undo hack. We just realized recently that upgrading Cygwin (which upgrades mintty) fixes this issue. The old Cygwin was using mintty 1.1.1 and the latest uses mintty 3.7.8. mintty 3.7.8 does not have this issue and both the old and the new were both using VIM 8.2. So the mintty side upgrade fixes the bug.