Effects/graphics "bleeding through" from the floor above
In this example the fire from a Militia Torchbearer servant's torch on the floor above can be seen floating around (see top right-ish of attached image.)
Its persistent and does not appear to depend on things like player action or camera angle etc (other than if the servant would be out of the shot.) The fire can be seen moving and bobbing along occasionally, corresponding to the servant's patrol pattern above.
I've seen other people mention similar occurrences with other effects like the high blood% glow from prisoners, Clairvoyance Hanging Lamps and others, but I had a good shot of this one.