we NOT intervene as soon as we know that a TCAS RA is taking place
Of course we only know of a TCAS RA by pilot reports. The controller will be ignorant of an RA for at least 5-10 seconds and possibly up to 20 or more seconds, that's plenty of time to issue an instruction which may be opposite to TCAS.
There seems to be more of a tendancy to give avoiding action in the horizontal plane only now, but will pilots follow this advice when a bank angle of more than 5 degrees can cause errors in TCAS ?