I think Tiax should trigger a comment to the content of the letter, but for some reason, the kickout dialogue triggers instead.
To prevent this, you can try setting SetGlobal("X#TiBhaal","GLOBAL",2) via cheat console before taking up the letter.
EDIT: wait, this is actual BG1NPC content, but related to his quest. I'll investigate...
EDIT2: turns out this is because of Tiax' quest: if the player let Belgin go in Tiax' presence, Tiax is supposed to leave but the dialogue (the one you encountered now) didn't trigger but it does now because the trigger is active.
Did you play his quest?