Also having this issue when creating our own MOs but it does not appear when I create a MO directly. Any ideas how to resolve? What on change method are we supposed to trigger for it to work?

Basically, there is an issue with the finished stock move, it's either the state value is wrong or the move is not there. To fix this, you first have to check if the destination location is not set to scrap location, if not; then check if that manufacturing order has a finished move if not you can trigger _on_change method to generate the finished move otherwise it has something to do with the state of the finished move. 

But this is generally a bug and our team will investigate the source of the problem in order to fix it permanently.

-- NEA