I believe the reason for this is explained in this talk
https://www.unrealengine.com/en-US/blog/take-a-deep-dive-into-nanite-gpu-driven-materials
The relevant section is page 23 of the PDF.
Essentially, it has to do with mipmapping and nanites manner of merging material rendering into a single full screen pass. Messing around with a nanite mesh’s UVs can interfere with the way nanite addresses this, causing a UV seam in manners that may not have been a problem with non-nanite meshes.
You may be able to solve it in most cases by manually calculating the correct mipmap in your material for the texture sampler.