The drone don't update their potential mining target list after the building is placed, which creates a few funny situations with mods that add regenerating resources.
Tiberium spreads over time, with new tiles spawning near existing tiles. But those new tiles are ignored, with the drones going to further points instead, allowing it to spread toward the depot, potentially out of range of the mining.
I have to repeat my request. Drones do run out of enough Tiberium they mine dregs as there's an entire field regenerated around them.
It doesn't have to be frequent, but every 3 minutes or so should be enough to keep up with the field deplention and respawn.