Sub-level, weird dark squares & z-fighting with nothing

I’m getting weird dark squares on meshes.

They’re actually 3d cubes, you can see it when meshes move, and if you align the mesh faces with the dark cube edge you get z-fighting.

You can see the squares on the left-most surface, and on the 2 cubes and cylinder you can see z-fighting:

(left cube is static, right cube is moveable, and the cylinder is stationary)

The meshes are in the persistent level (which has Force No Precomputed Lighting), and there’s a stationary directional light in a sub-level (and also a stationary sky light). I build the lighting in the sub-levels themselves.

(I also placed a cube inside the sub-level, and it also has z-fighting. However, when I open the sub-level by itself, the cube is fine.)

Dark squares don’t appear in un-lit view.

Same persistent level, different sub-level (w/ different lighting and objects far away): no dark squares.

I’m thinking it has something to do with cubemaps? Maybe it’s the reflection captures in the sub-level that actually envelopes the persistent objects… (although it happens on materials that are completely rough, if that changes anything)

Some other threads with what seems to be this problem:

([forum post][2] for cross reference)

wasted a day upgrading to 4.27 - didn’t help…