It is because the structure is not linked to the base it is at, so its looking for supplies nearby but can't find any, even tho the base has 4000 supplies

Good catch!

The E_VehicleMaintenance_S_FIA_01_1 (7542.708 166.844 4201.801) is not included in the composition CP_LevieMilitaryBase

Its also bugged for every single light vehicule maintenance point I think.
-Need to be tested-

Still not fixed in the newest patch btw

It's not related. You can build a new one, and it still won't see the base's supplies

Workaround - build them next to supply depots

Sample of workaround:

Placed close to the main tent