I reported this for the skittering caverns when I played the last weekend of the test.
To me it seems that there is a lighting switch if you go a bit into the cave from the opening. That is where the lighting changes but also where the late spawn starts.
Oh, yeah, I absolutely expect them - Sinner’s Refuge I know exactly who’s going to be missing and approximately how long it will take for them to show up (which is why that’s the one I videod - just so easy to predict exactly who would be where). With the other caves I haven’t got it down to quite the same level of accuracy, but I know it’s coming and have a reasonable idea of when.