Some times polymorph seems to randomly cancel after only a few seconds. The enemy (PvE) will have sustained no damage to cause this effect. Not sure what’s up.
I could be wrong but I believe this is normal. It used to break early sometimes.
I searched around but couldn’t find anything so I don’t know
Polymorph duration is up to that amount but not guaranteed. The enemy has a heartbeat which can randomly break at any time. The closer to the max duration advertised for polymorph the greater the chance the heartbeat will break the morph.
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Ya have a source for that?
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Uh yeah. Blizzard
This is true 
I also believe some magic resistance mobs will also increase the chance of it breaking early.