Ah k, must be capped at 100 then.
I did do a quick test, I put a spiral of 2,000 mirrors, sides/roof/etc. Not one of them turned black. Are you sure you dont have something black in the scene where its leaking through such as a black background? Can you try set your environment to 100% white and see if its the same? Just a thought, you could be entirely right since I dont have same angles you do.
It's not environment, it's
exit color. You can find it in dev/experimental rollout. You can change it from black to any colour you like, but it's solid colour and might not help much in scenarios like described in this topic. I had submitted request to replace exit colour by environment many years ago, unfortunately it was rejected by the devs.