Upgrading project to 5.6(.1) causes material slots on previously imported meshes to break/swap. Seems Nanite influenced as well.

Development Asset Creation
1

Hello, we’re experiencing an issue with upgrading a 5.5.4 project to 5.6.1. Upgrading causes meshes previously imported (possibly only via Datasmith for C4D) to swap or destroy material slots when upgrading.

You can see here, this lid, when upgrading to 5.6.1, the material for the painted wood is missing and is replaced by the metal which existed in the other slot.

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However, if we disable Nanite on this mesh, we see that the original material slot is now broken:

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We have also tried disabling Nanite in 5.5.4 before project upgrade, and then upgrading to 5.6.1 without any Nanite static meshes. This creates an alternative scenario where the non-Nanite mesh is fine, and displays correctly:

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But when enabling Nanite on that mesh, the issue returns:

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We’re really not sure what to do here, as there doesn’t seem to be any other pathway from 5.5.4 to 5.6.1 without a simple project upgrade, and this is happening from the upgrade itself without any other changes made to the project.

This is a blocker for us, because we’re unable to move to 5.6.1 without this issue happening, meaning that our art assets are stuck in 5.5.4. It would be an enormous job to reimport all assets fresh in 5.6.1.

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Another update to this, it seems that meshes that once had two “mesh sections” in the static mesh editor, now have three:

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And when enabling Nanite on this mesh, it returns to two materials, but for some reason takes the duplicate section instead of the original second section

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I haven’t found a way to remove the extra section in the non-nanite mesh yet, but I suppose if there was a way to delete the spare section, it might preserve the original material slot order when turning Nanite on?

Section 0 (the newly added/false section) contains nothing:

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Section 1 (the original metal material slot) contains the expected area:

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As does Section 2, showing the painted wood material:

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So for some reason, an excess Section 0 is being added between 5.5.4 and 5.6, and when Nanite is enabled, that Section 0 is removed (because it has no mesh data assigned to it) but it breaks Section 2’s material slot as a result.

I’ll keep trying things.