It seems that the bug was caused by how the sprites for the mutant were serialized and saved on browser versions. They also differ browser to browser? Who knows. Wrote a workaround, found out via using the profiler to reach the error stack. 

...

<sprite> at SpriteRenderer_set_sprite</sprite>

<sprite> at Mutant_Load</sprite>

<sprite>at Mutant_Start</sprite>

Either way its painful i resolved this after the jam, hopefully this wont impact the playtesting as much.