I am on UE5.2.
If your project is set to EV100, maybe you set the Sun to 100,000 lux - at least that´s the Sun´s lux value on a sunny day, at 12 o’clock.
In this scenario, the Sun´s Indirect Light Intensity is not doing anything even if you go up to 1 000 000 000 values. And I don’t know why.
But if you set the Sun to the “old school values: 5 - 15… lux”, with the project at EV100, Indirect Light Intensity it works.
So, it doesn’t seem to have to do with the project being set to EV100, but with the Sun´s lux value.