Thank you for your prompt reply.
That was my first thought, though I checked and in the bits of the original model which weren’t in UCX layers, there is only a single (single-sided) polygon in the relevant areas. And there is no apparent difference in the original model between areas where this z-fighting is happening, and areas where it is not.
On further reflection, I’m not sure this happened prior to 4.11 (or possibly 4.10). I’ll try re-importing the mesh to see if that makes a difference.