Figured it out.
Accidentally.
Due to the fact that I used nDisplay (херня for rendering multiple screens), the lumen was incorrectly considered. More precisely, each frame was recalculated for each screen, and since there is formally one cache, each calculation was overwritten by another. It helped to remove NDisplayRenderActor, wait for the lumen calculation and return it back. How to do this during rendering - I do not know.
This is basically a “solution”, but in fact - no.
PS. …checked it out. Each camera from NDIsplay uses its own lumen cache during rendering, which is why this bug is only in the editor