Render distance: 16 chunks
Distant Horizons: No
Iris: 1.7.2

Issue:
While underground, distant areas with no light source appear to have daylight. Setting the time to midnight removes the light. Approaching those areas in the middle of the day causes the light to dim, while the area I left behind lights back up.
Lowering my render distance back to 12 chunks does not appear to resolve the issue.

In the screenshots, I am standing directly beneath the only opening in the cave system.

RTX profile:

Shadows profile:

Colored Light profile:

set Lighting > FloodFill > SkyLight = None
or set shadow samples to zero/disable LPV_GI if you're on release 0.11

should also be disabled in current in-dev if you want to grab that (download code button on github home page).