So it looks like Precision split pickup halves have opposite windings and opposite magnet polarity. connecting them in series allows hum cancellation with all four strings in phase. The traditional Mustang pickup halves are all wound the same direction, with all magnets the same polarity. Connecting them in series “tail-to-tail” Effectively cancels hum, but with opposite signal phase between the halves.
I suppose Fender was saving money building all of the Mustang pickup coils the same, for what was conceived as a “budget” or “student” bass.
I’ve never heard or read of anyone bothered by the difference in phase between the E-A and D-G strings on the Mustang.
The new American Performer Mustang obviously required a redesigned split “Mustang” pickup to allow combining with the Jazz bridge pickup.