I noticed that when I was around when the structure got damaged, then the construction drones deployed and repaired the structure. However, if the structure was already damaged and I just happened to come by, they did nothing, even toggling the drone deployment doesn't help. This is especially annoying since they don't deploy out of a car, and if you hit something and get out, then they still don't repair the car nor the structure.

How many things on your map are damaged?

Not many, some walls and turrets or the power pole I just hit and my car.

I've created a short video to illustrate the behavior: https://youtu.be/2Qc1aWW9_P4
Here is the save https://www.dropbox.com/s/blre4ay6dofou6x/NewOldSchool.zip?dl=0

This seems to be fixed with factorio 0.18.22

Is this fixed on .22? I haven't updated yet until the rest of my playthru's mods are updated.

I've noticed this before but wasn't sure if it's drone's or K2's doing. The drones don't repair anything newly placed. Take a turret. Shoot it, drone comes out and repairs it (like your video). But take an already damaged turret already in your inventory and place it down. Drone won't come out to fix. Using construction bots, placing a damaged structure and it will repair it.

I play with https://mods.factorio.com/mod/inventory-repair so I can't confirm that newly placed damaged items are repaired, but with the update they repair stuff damaged by car or that was damaged out of sight.