I've seen that, too, but to that point, when I screened a few prints on a vintage projector, with vintage bulb and screen, which apparently had lower reflectance by default, they weren't visible. It's been my assumption since then that improvements in screen reflectivity and/or increased luminance output was the culprit, but this doesn't explain your anecdote from "back in the day." One thing is for sure which is that during the development of the shots, whatever they were screening the tests on was not exhibiting the artifact or they'd have compensated.

Where does that leave us? Certainly I have been setting my garbage-matte-black-point nice and low in the P3 curve...!