The only way to reproduce this bug is when your Dekan is in Humanoid form and trying to use Evolve or some other skill that casting it is instant and having a huge amount of monsters poking you. While in Evolved mode casting a skill while it expires and having the same stupid amount of monsters poking you will result in staying in Humanoid form with Evolve mode passives still active.
That is the only way and in my experience I've only had this problem in pservers, here not that i remember but it is possible cause it hasnt been fixed yet.