This was the case when the slot index in the active object is greater
than the available slots of other selected objects.
Code was trying to avoid searching all materials by using
`BKE_object_material_get` to get an appropriate index (an addition to
D4441 added in 6b39dc7672). That function has the behavior of clamping
if a target index is greater than the available slots (for good
reasons), so we cant rely on the slot index being the same if
`BKE_object_material_get` finds a material. So in essence, this is not
what we want to use in this case.
Now use the much simpler `BKE_object_material_index_get` to get the
index [this might be less performant in certain scenarios, dont think
this is critical though and correctness should beat performance here]
Pull Request: https://projects.blender.org/blender/blender/pulls/123718
296d05060d