Ok, I think I see the difference. If I type text in a static text object
and then click outside the object, then click the static text object to change some of the text, then click Ctrl+Z, only the text I
just modified reverts back to the original.
In your case, you must have typed some text, then changed what you typed
without clicking outside of the object. In that case it makes sense - your original text change was never "committed" to the object, so it reverts back to the the "default" that appears in a next text object (in my case it says "Text").
Make sense?