Environment / Asset
- Unreal Engine 5.6
- Static Mesh with N material slots and LOD0 having more sections than slots (e.g., 7 slots, 14 sections), so multiple sections share the same Material Index.
- (The issue can also be reproduced when the number of slots equals the number of LOD0 sections.)
Steps to Reproduce
- Open the mesh in Static Mesh Editor.
- If the number of Material Slots and LOD0 Sections differs, UE 5.6 automatically remaps sections to make the counts match. (This Remapping is also not correct)
- To free a material slot for deletion, in Details → LOD Picker → LOD0 → Sections, pick a section that uses the slot you intend to delete and reassign its Material Slot to another slot already used by a different section (creating a deliberate duplicate). Repeat until no section uses the target slot.
- Delete the now-unused Material Slot in Static Materials.
- Observe: after these reassignments and the deletion, UE 5.6 auto-compacts LOD0 sections and misbehaves:
- The section count snaps to the number of material slots.
- The wrong section is removed (often the last one) instead of the duplicate you just reassigned.
- If reduced beyond certain counts, duplicate sections remain and cannot be removed, or unexpected sections disappear.
- In some cases this leads to an engine crash. (Nanite is enabled in our project; we suspect an interaction between Nanite rebuild and the automatic remapping may contribute, but we’re not certain.)
5.5 vs 5.6 Behavior
- In UE 5.5, the editor did not automatically rebuild/compact sections, so multiple sections sharing the same material index remained. This hurt performance (extra draw calls) but didn’t reshuffle/delete the “wrong” section.
- In UE 5.6, the editor auto-remaps sections to match the number of material slots, which is where the incorrect deletions/reshuffles (and occasional crashes) occur.
Project Context
We primarily use level props purchased from external vendors. Each asset arrives with its own material setup, which severely impacts performance and visual consistency. Our workflow is to open assets and standardize materials—removing unnecessary ones and remapping the rest to our master materials—directly in the editor. That process requires reassigning section materials and deleting freed slots, which triggers the behavior above.
Request for Guidance
Do you recommend a stable workflow or tooling for this scenario in UE 5.6? We have a large volume of assets, so exporting to a DCC, editing one by one, and re-importing is not feasible.