I was testing out an HDR image Firefly render by subdividing (to level 2) the Environment sphere that came with a Jefferson AF product. I added a random figure (Bertha the nurse), and I got the splotches. All lights off. GROUND visibility and light emitter off. My cure was subdividing.

Render settings. These are the PBREmulatorBetter settings with HDRI check added.

Splotches on shoulder.

Lowered render setting IDL from 15 to 1. Dimmer splotches on arm.

Subdivided dress to level 1. Reset IDL back to 15. Dimmer splotches on arm.

Subdivided dress to level 2. No splotches!

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W11,Intel i9-14900KF @ 3.20GHz, 64.0 GB RAM, 64-bit, GeForce GTX 4070 Ti SUPER, 16GB. 

Old lady hobbyist.

All visual art or fiction is "playing with dolls."