Ok I can reproduce this in UE 5.1. And if I click “Platform Stats” while the material is open, UE reports:
Which is very strange because “Used with Skeletal Mesh” is disabled, so I’m not sure why UE is concerned with any skinning. It may be a UE 5.1 bug, as it doesn’t seem to happen in UE 5.0.
I’ve opened a ticket on UDN to ask for advice from Epic:
https://udn.unrealengine.com/s/question/0D54z00008dPGb6CAG/too-many-texture-coordinate-sets-defined-on-gpuskin-vertex-input-in-ue-51-but-not-earlier-versions
Kevin