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"Join Meshes" changes the material slot assigned to faces of the original mesh
Confirmed, NormalPublicBUG

Description

System Information
Operating system: MacOs BigSur

Blender Version
Broken: 2.91.2

Short description of error
See video:

Exact steps for others to reproduce the error

  1. Create Mesh ONE with 2 material slots, with the same material, each slot assigned to different mesh island in mesh.

So faces on mesh islands have different material_indexes, although both use the same material (in different material slots)

  1. Create Mesh TWO without any materials
  2. Join Mesh TWO into Mesh ONE
  3. Check material indexes in resulted mesh. They wil ALL point to first material slots.

Faces, that was linked to second material slot initially will be "relinked" to first slot, so theier material_index attributes are altered/messed

Expected behaviour:
Faces linked to second material slot initially should stay with the same material_index, join action has no reason to change them

Event Timeline

Germano Cavalcante (mano-wii) renamed this task from "Join Meshes" action messing up material indexes on resulted mesh to "Join Meshes" changes the material slot assigned to faces of the original mesh.Tue, Feb 16, 4:00 PM
Germano Cavalcante (mano-wii) changed the task status from Needs Triage to Confirmed.
Germano Cavalcante (mano-wii) changed the subtype of this task from "Report" to "Bug".