Sorry, I'm late. Yesterday I was busy trying to upgrade my Ubuntu 17.04.
I saw you already added the note at the bug report.
Have you tried TSynEdit? I wrote myself a small programs to help me organize my notes. I got several problems with TMemo, one of the issue was undo/redo. So I used TSynEdit. You can think TSynEdit is a more powerful version of TMemo, the syntax-highlighting can be disabled so it will give the result as a TMemo. Maybe you can try it.
http://wiki.freepascal.org/TSynEdit
Thank you for your help, I have tried TSynEdit, but it can‘t enter Chinese, so I didn't continue to learn it.