I don't understand the inner workings, but I get the impression that there is some fuckery going on when keeping the same distance to the screen and heavily diminishing the amount of light reaching the eyes. There may be a threshold (varying among individuals) beyond which the pupils become too open to clearly focus on the text.
I have a slight photosensitivity and most outdoors daylight conditions are too much for my naked eyes, but many dark UIs are too high or too low contrast and feel worse for extensive reading than their light counterparts in a screen matched to a moderately illuminated room (and this also also includes changing screen settings to match environment light conditions when switching between light/dark themes, not just trying with the same settings).
This leads to a life where my screens are stoically dimmed way past their sweet spot most of the time. Colors suck a bit, but at least I can always focus on light UIs without strain and it avoids text persistence (burn-in) in the eye.
Lately I'm trying GitHub Dark (not Dark Default) in VSCode for dim environments at night; it's the lesser offender while retaining some legibility/contrast. Dark Discord seems bearable for a while, too.