I was trying to reassign Maul while shapeshifted near the end of the duration and ended up assigning it to my human bar. I am able to cast Maul as a human (which results in a fairly humorous animation).
Edit: After testing it, all you have to do to reproduce the bug is have the skill swap menu open when you shapeshift expires. You can then assign any wearbear skill to your human skill bar.
I just encountered this as well. The menu also stays up if you use the “Human Form” skill to shift out of form and then assign a transform skill to your bar in human form.