
So that's the story. After much headache I finally isolated the strange behavior to buffer chests simply not caring that they're turned off. They still report and deliver all their content to logistic networks despite showing to be shut off (which is probably why it was so difficult for me to track them down).
But fortunately, it's easily and 100% reproducible. I did a little test map and it works exactly the same. All chests (with the exception of green buffer chests) that provide their contents, red, yellow, purple, no longer provide their contents to the logistics network when shut off by a circuit. The funny thing is that the green chests still report "Disabled by control behavior" and display the little red light on the circuit panel (like all the other logistic chests), but their content is fully visible on the logistics network all the same and will be freely accessible to robots.