Already opened a ticket and posted it to the official bug forum, but figured some of you should know in case you'd like to test it out before you get boned like me in Arena >.>
The Hunter ability Master's call I have found as of today is not working as intended. Although MC seems to break the initial slow/root, any slows applied afterwards at least in the form of crippling poison and chains of Ice are not stopped from being re-applied.
After further testing, root effects are protected for the duration as stated, as is a mages frostbolt, BUT the frostbolt debuff is shown and my character turns blue as a result, despite not being slowed by it until after the Masters Call wears off.