I haven't been able to reproduce your issue, moriador, although my largest render was 3000px. Will try a bigger one, but in case I'm missing something, this is what I've got in my scene and render setup:

SCENE ELEMENTS

BB's EnvSphere

1 - one sided square primitive set to 3 ambient.

1 - displaced ball primitive with SSS applied.

(2 lights with diffuse and specular set to black)

RENDER SETTINGS

I haven't delved into D3D's advanced render Firefly GUI. In the regular render settings:

Casts shadows, SSS, Raytracing all checked; Use displacement on.

Raytrace bounces: 1

IC: 50

Indirect Light: Quality: 10

Pixel samples: 4

Min shading rate: 0.4

Min discplacement bounds: 0

Gamma correction: 2.20

Post filter: box / 1

Am I missing anything?

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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