I had a similar observation after patch #9. The only difference between “RT off” and “RT custom” seems to be weird lightning flashes on the latter setting. It seemed to me like just another Darktide bug so I didn’t bother to deal with it other than commenting on a bug report about lightning issues that got acknowledged:
https://forums.fatsharkgames.com/t/there-is-an-issue-with-the-lighting-in-the-game/79157