Thanks for all the comments, folks!

1. Issue is not irradiance caching. All the test renders have been set at either 99 or 100.

2. I've tested with and without AO, with and without raytraced shadows.

3. It's not a homemade texture - well, it's a well-respected merchant resource texture (PureHot from DAZ - by Sarsa, I think) underneath, although that's well-disguised by my own work. The texture definitely covers all seams. In any case, I've had this with numerous unrelated textures.

4. The morph may look "odd" because it's a custom sculpt that is unfinished under the chin. But I get these issues, as I said, quite apart from this character, and in different parts of the body. (Right now I'm running a render with the morph at zero just to check.)

5. No, this isn't all happening with the same bump map - I was experiencing similar issues prior to working on this character, and the bump map here is completely custom.

I'll try setting the face material to zero on the bump and see what happens.

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Hardware: AMD Ryzen 9 3900X/MSI MAG570 Tomahawk X570/Zotac Geforce GTX 1650 Super 4GB/32GB OLOy RAM

Software: Windows 10 Professional/Poser Pro 11/Photoshop/Postworkshop 3