[Edi]

Admittedly, I have not tested that, but went from memory of how I've seen shapechange work previously. Basically battlefied shapechange causes the unit to want to revert back to the form it had when it entered battle. Wit things like Jaguar Warriors or Black Sorcerers, the shapechange is always involuntary and the return to the original shape is through the #firstshape mechanic.

For units with full shapechange, the shapechange command is triggered. In most cases, such as the dragons, it simply switches the form back and forth. With the Jotun Skratti, it cycles into the next form and drops the gear. In that case it would need to cycle twice or skip over one form with some kind of offset factor, which does not exist in Dominions 3.

Therefore it's a feature, not a bug.