Hum in parallel position on a 97 '52 reissue Tele

My 1997 '52 reissue copper Tele is wired to a 4-way switch and both, the 2nd and 4th positions (neck+bridge in parallel and neck+bridge in serial) hum as the single neck or bridge pup alone (where normaly these position cancel the single-coil hum).

Anyone knows why this is happening (I have not replaced the original pups)?

The pickups wiil only be hum cancelling in positions 2&4 if one of the pickups is reverse wound/reverse polarity to the other (RWRP). That is NOT the case with Fender Original Vintage Tele pickups such as are in your 52RI.

Fender pickup sets which do have that feature are Nocasters and Custom Shop Texas Specials. A set of those or even one of those bridge pickups with your Original Vintage neck pickup will give you hum cancelling when in combination.

It doesn't have RWRP p/ups. No such thing in '52. If you want noise cancelling you'd have to change the B magnetic polarity w the move past 2 neodyniums trick, ground the base alone, and use the Black wire as hot and ground the White for 2 and 3.

Thank you very much for the info!