I understood it to be that Peter had accidentally damaged one of the two coils in the neck pickup, shorting out some of the windings, or making them in some way offset with each other within the same pickup. Alone it sounded fine, but could not fully defend itself ( in girth ) against the bridge pickup in the middle toggle position.
As already mentioned , only the 'Mighty Tim' knows the real answer.
( Oh, and a belated welcome too ! ) :)