I'm also seeing this issue. In my case, I have a multi-level assembly and I want to create a Model Item for a sub-assembly. I drag the Model Item widget onto a part that belongs to the sub-assembly and everything is OK, even if I save the project, exit out and then come back in again. But then I edit the properties of the Model Item and change the Component Occurrence to select the sub-assembly. At this stage it looks OK on screen, but if I Preview, or exit and come back in to the project, the whole sub-assembly has moved to a new location. as shown:
If I remove the Model Item widget, then the sub-assembly immediately moves into the correct position:
It looks to me like it works OK for individual parts, but it's broken for sub-assemblies.