Have a look at the characters with special functions in format strings:
Esp. check the square brackets.
Yet, I would check the purpose of the TEMPO field as that is, unlike the standard field BPM, a user-defined one that probably not a lot of players will display.
So perhaps it is wiser to copy the contents of TEMPO to BPM (if they have the same context) and then use only BPM and delete TEMPO.