thats due to Hot Running into the Bridge from the copper coated elevator base plates adjusting screws,
take and clip away the bare metal ground wire from the base plate (only) ( not the black cloth wire from the eyelet ground )
~next run a separate ground wire to that base plate and it should all be back to normal -
its like the neck - when you switch Ground wires to "hot" you must take out the ground jumper wire to the necks metal cover or in your special case the "base Plate"
usually we only reverse wires at the neck pickup on a Tele-
this is because - sometimes the bridge pickup will not stop grounding out to the base plate
- as it was- the old Fenders will sometimes ground the magnets to the plate - this happend from those pesky partial shorts in the start wires of the fine wire coil itself.