Are you, by chance, using Dectorio's concrete grid for your sidewalks? At a glance through the code (which does some complicated things with collision handling to make the drones stick to "road tiles"), I'm not seeing anything that should make the drones default to using base game concrete, brick, etc. pavement for roads. But the default whitelist does include "^dect%-concrete%-grid$", which activates use of that tile from Dectorio. That's in the mod startup settings.
Beyond that, my best guess would be the way they're handling "layers" applying those collision rules.