A pickup can soak for days after it's stopped bubbling, and then the second
it's in a vacuum chamber, it will erupt in a sea of air bubbles,foaming the wax
like cheap champagne...and all that air is coming from someplace, places inside
the coil that were not waxed.

NO amount of soaking in hot wax can do more than penetrate merely the
innermost, and outer layers of the coil.- that's enough for most
manufacturers and most pickups, most of the time.
And, most often when an old pickup squeals, re-waxing it will re-melt
and solidfy things enough to stop the problem. -usually.

But, the ONLY way to fully 'Pot' any guitar pickup, is with a
vacuum/pressure system (like we use). It's too expensive and time
consuming for big manufacturers and usually not necessary, unless
it's YOUR pickup that starts squealin'

The proof is to cut apart the coil, and then one can plainly see that
coils soaked in wax, even for 2-3 DAYS, are still mostly un-waxed,
while a vacuum/pressure potted coil will saw apart like one solid
piece of copper.