It is permanent; you'll have to reload the save just before that.
All of the polymorph traps were suppose to have been removed when the effect was made permanent, but 2 were missed, in the Verdistis sewers (
map).
Level 3 of Evade Traps prevents the polymorph trap from being triggered when you open that chest, or a spiritual resistance of 16 or more will protect from the effect.
