Hey Mete,
I just manually modified the text for an existing annotation feature i did not use an expression to generate the particular text. Can you check in your advanced callout properties, that you have Use Custom checkbox unchecked. You also mentioned that you changed the TextString field i just want to make sure that you used attributes window and changed the text in the annotation tab because when we paste it in the attributes tab it sometimes looses the formatting and line breaks.
It would help me if you could post a db with your annotation class so that i can check what is going on.
Thanks,
Amit