Pixelated patches on a UDIM mesh usually mean that some UDIM tiles are being shown at a very low mip, rather than the model itself being broken. Things to check in UE4:
- UDIM textures must be Virtual Textures. Enable Virtual Texture Support in Project Settings > Rendering, then make sure the texture is imported/marked as “Virtual Texture Streaming” and the material samples it with a Texture Sample whose type is Virtual Color (or “Virtual Normal” etc. for normal maps). A mismatched sampler type will give garbage/low-res results.
- Tile pool: in the console run r.VT.Residency.Show 1 to see the VT physical pool. If it’s full, pixelation shows up on random tiles. Increase the pool (r.VT.PoolSizeScale 2, or the VT pool size in Project Settings > Rendering > Virtual Textures) and see if it goes away.
- In the texture editor, check LOD Bias and Never Stream aren’t set oddly, and the texture group isn’t forcing a small max size. Also confirm all UDIM tiles share the same resolution, since a lower-res tile will look pixelated next to the others.
- If it only happens in some areas, test with r.VT.Borders 1 and r.VT.AnisotropicFiltering 1 to see if it is a filtering/border issue.
If it still happens, post a screenshot of the material graph (texture samples) plus the tile naming (1001, 1002…) so it’s easier to see.