As currently implemented, Blender sometimes incorrectly sets active/render
UV maps during the "realize instances" operation. This also comes into play
if you try to modify the mesh with geonodes: e.g., if you use geonodes to
split a mesh into two pieces, transform one of them, and then "join geometry",
join operation internally performs "realize instances", triggering the same bug.
The root cause is that "realize instances" copies both UV map layers and the
active/render layer index, but it reorders layers during this process, so the index
that was correct in the original mesh is now wrong in the modified mesh.
This patch corrects indexes by explicitly checking layer names.
Pull Request: https://projects.blender.org/blender/blender/pulls/116452
