As the title, if a vehicle's grid is filled by an inserter, the Electrical Transformer doesn't do anything.

Should be an easy fix (adding event trigger). Which mod do you use to make inserters install items into electric grid?

I didn't realise there was a mod for it. The inserter just... did it.

I've checked vanilla + this mod only, and inserters do not put anything into a grid. So I'll need to know what mod does allow, and check how it works, what's inside its code.
This mod uses on_equipment_inserted trigger, which doesn't care what put equipment inside the grid. If the other mod uses some sort of scripts that bypass the trigger, then the only possible fix would be to modify other's mod code to call on_equipment_inserted event

I can only guess then, as it wasn't me playing at the time.
Possibly "Equipment Gantry" having weird interactions?
(They were playing with Spaceblock: Trains and Warehouses)

Fixed this issue with "Equipment Gantry" mod (tested it). Should work in version 1.1.3+