This was caused by a hash colision caused by an incorrect
auto conversion from `Object*` to `ObjectRef`. The `ObjectRef`
would then not contain any instancing data and would create
the same hash for the same particle system on each instance
(since only `foreach_hair_particle_handle` was constructing
`ObjectKey` from an `Object*` it did not affect the instance
itself).
Marking the incriminating constructor `explicit` to avoid
more issues. Changing the `ObjectKey` creation for psys to use
`ObjectRef` fixes the issue.
Pull Request: https://projects.blender.org/blender/blender/pulls/140544
234ff21a7b