What's causing this weird stretching?

Easiest said in pictures. I can probably cut it right down to the troubling corner if you need a blend (It’s part of a much much larger scene). It’s the only place it happens.

It’s a single, much larger face with a subdivision surface modifier, and the texture is entire procedural.

And for good measure, the UV is clean (I’ve also checked connected points)

It’s the only place on the model it occurs, and with subd turned off, it goes away. Not using adaptive subd (though I tried, and it makes no difference).

EDIT: After further testing, the higher the subdivision the worse it gets, but only evident with Catmull-Clarke. Simple subd doesn’t do it.

OK, here’s a stripped back blend.Texture_issue.blend (2.92 MB)

Since the leather texture is fully procedural, you can use another coordinate system instead of UVs (for example ‘object coordinates’)…

If you plan to use image textures later, than probably you’ll need to change the mesh topology to become more coerent for the subdiv.

I find using Normal mapping works on the sample area, but I’ll have to test on the full scene, which is a 20 - 30 minute render (per frame).

Nevertheless, that doesn’t explain what is going on.

Nope. It has to be UV. It’s just wrong any other way.

What you have are some distorted quads, that the subdivision algorithm doesn’t deal with ver easily. Having a better topology can fix this.
And using the normals as a coordinate vector is not the best solution for this case, as the texture will squeze/expand depending on the surface curvature; Creating diferent results if the surface is plane or curve.The generated coordinates can be used, but require some scale tweaks as the bounding box of the object is not really a box. ‘Position’ makes the texture fixed at world coordinates, and it can be used if the object is not moving; so the best is the ‘Object’ coordinates.

Edited: if it has to be uvmapped, then you need either change the topology for using subdivision or apply the subdiv and unwrap after (or something like that)… try keeping quads following the surface qualities, not just for filling areas…

The only quad I can see that may cause an issue with subd is this one:

Nevertheless, even if I make sure that it is perfectly aligned (all 4 vertices aligned on their own plane) it doesn’t make a difference. Perhaps catmull-clark simply doesn’t handle this kind of geometry very well?

Nevertheless, I can tweak the scale of the texture to work with Object mapping, thank you.

EDIT: Nope, not that one. Joining the centre vertices to convert to tris doesn’t help. Converting to tris then dissolving the outer two so all quads again doesn’t help.

Whatever I do, that (and only that) bottom corner of the larger quad is distorted. Conundrum.

also, having lots of uv seams in that area doesn’t help. It’s difficult to interpolate uv’s when there’s gaps in their coordinates continuity…
here’s a screen shot with a new uv layout (just for the piece you supplied):

Thank you. I forgot I’d used smart unwrap. Re-unwrapping properly has fixed the issue.