How to reproduce : 

In a rich text (in back office or front edition), insert : 

1/ A text (a paragraph of lorem ipsum for example). 

2/ Set the cursor at the beginning of the text

3/ Select "Titre 1" as style => All paragraph got "Titre 1" style. 

4/ Click on Undo action => Nothing changes KO

*5/ Click on Undo action => * All paragraph is updated to previous style. OK

(A single click on Undo action should restore previous style).