Possible bug? Unity forces transform Y scale to 0 when reordering in hierarchy, disappears if you try to fix

I have a model from Blender. I import it by dragging it into Unity. I add it to the scene and choose "unpack completely" to restructure it. Once I do that, some of the meshes in the model do something weird...

If I create an empty as a child of the model, and then move a mesh to be a parent of that GameObject, Unity forces the mesh to have a transform Y scale of 0. Manually changing this value in the transform part of the inspector to anything other than zero makes the mesh disappear completely. This happens even if I try to move the mesh into the world space (and give it no parents).

This happens only for specific meshes in the model; others are fine. Other models do not have this issue. I have no clue what's wrong or even where to look. My suspicion is this is Unity-related and not a Blender file problem... after all, the model works fine when I don't move any of the unpacked assets around.

The video shows what I mean. Link here

Any ideas how to prevent this?