They do appear to have a priority system but I do think that vandalised item fixing is bugged .. I had one mechanic do it once (honest.. image attached) .. and left the one next to it to go wandering....
I believe someone else posted about this and someone has reported it as a bug.
Priority system wise:
Habitat Repair is their primary concern .. if you don't want that to be their priority .. remove it from their list.
Facility Repair ( Power ) is their next target ... I usually have at least one concentrate on power and water alone.
Facility Repair (Water) is their next target ..
I unfortunately lost track of my science project target when my back was turned while he was in the staff room rofl. So can't confirm the situation beyond that until I test it again on the penultimate scenario which has so many damages to fix from the get go .. its a great testing ground for mechanics rofl.