I suspect that roof tile was hit by something other than shrub. Shrubs only hit frames and armor, I think. How fast were you driving? I think the car takes more shrub damage the faster you drive… Also try to drive straight and not in an angle. Angled driving can expose the sides and inner tiles to collisions.
My vehicle has 5 shredders in the front. Behind them is just heavy frame bumper, armored. The shredders don’t take shrub damage but the bumper and the armor do.
I’ve found it wisest to just drive under 90km/h (off-road) and check the vehicle after every trip.
You could also install roller drums to the front, which I believe are beasts and they hit and crush everything, although being heavy and bit rare to find.
Do install armor, too. You should be able to install armor on any vehicle tile. Steel plating, military composite armor, etc. If you have tanks enabled, they provide virtually endless amounts of military composite. Security vans in turn have steel plating all over them. You can also craft steel plating. Protect the precious tiles first.
I don’t know exactly how the damage is applied but without a doubt the armor absorbs the most hits. If no armor, then it’s a crapshoot. Shrub hits are probably shared between frame and armor. And here is the lump of salt you should take all this information with: *