I actually thought I reported this issue already, but couldn't find it so I'll report it again just to make sure it's addressed.
There is a very annoying little issue with text editing and using the Undo/Redo. Let me lay out the steps so you understand exactly where it happens:
Create a line of text as a paragraph, heading, etc., doesn't matter what kind, it happens in any text editing within elements (not on custom code as that's not set up the same).
Double click that text to go into edit mode and place your cursor where you want to change things. Edit some of the text, but don't click out of the text box and don't click anything else at all, just edit the text and leave the cursor right there waiting for more text editing.
Hit Undo ....
This is where it breaks the Undo Redo setup in a sense. Instead of undoing the text edit as you would expect it to, it's undoing the last thing you did "prior" to editing the text. This is most likely happening because you have to click outside of what you're editing (no matter what it is, css attributes, text boxes, etc.) in order for that change to take effect.
I'm guessing that in the case of text editing, the app isn't seeing the changes as having been created yet because you're still editing them. But ... this is most definitely not a typical way for Undo to work. If you're editing text, it should be undoing and redoing whatever it is you're editing in the text even though you're still in it.
Anyways, that's it for the most part. Undo doesn't pertain to currently active text editing and undoes the previous action that was clicked out of last (clicking out of meaning you clicked on something else on the page, or in the code etc. and made that change you just did activate). Redo will redo whatever it undid, but of course you have no idea what that is since you're in a text box .... and of course the app doesn't follow your undo changes (another idea/issue that I already suggested/reported).