In a guitar pickup, the magnetic field is "normally" stronger under and around both E strings. A short bar makes this magnetic field... laterally shorter. Hence a wee bit less bass mostly. High frequencies are less affected since the resonant peak of the pickup is in the high range.
But that's with all other things being equal and magnets are not always equal , by nature. They can vary in their composition and measured magnetism according to the foundry where they come from, from batch to batch and even individually, at least when they're rough cast. Sintered ones seem more consistent.
Reason why I test them with a lab Teslameter AND don't hesitate to swap the mag of a pickup until it sounds to my taste, regardless of the theoretical specs involved. Half a dozen of times if necessary. YMMV.
Out of the usual shops (SteMac, Addiction-FX), let's name Throbak as a possible "boutique" source of AlNi(Co) bars made in USA. Most of those sold elsewhere come from China.
https://www.throbak.com/alnico-magnets.html
[NOTE - Please, let's not argue again about how ThroBak describes the tonal effect of his magnets: what he says obviously comes from what he noticed with his own pickups. Other pickups may react differently to his magnets, depending on the specs of the coils, slug and screw poles + keeper bars involved.]
FWIW.
