Well, it's not 'just' about the winding order... I suspect that your 'fix' above is just a case where that particular order happened to work.  If you simply reverse the winding order of the existing vt references, they'll flip the texture around in various ways (but still use the same texture space for that polygon), but the artifact still appears.

I'd also comment that, IF you were using Bump Maps (or displacement, etc), then the shading would indeed depend on and need to reference the UV values, but even then, I would think that it was just to look up a value (bump or displacement amount) for that pixel - the UVs (in and of themselves) don't contain any 'normal' information (aside from the case of using a 'Normal Map' - but even with that, the 'shading information' is the normal map pixel value found at that UV location, not the UV location itself).

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