The problem arises when we try to install a humbucker in a cavity that previously housed a P90.

It also depends on what you like and what kind of sound you want.

. .

Yup they're both neck/middle mini hums.

Drop the neck and move the bridge up a hair and see if that solves the problem.

I put together a double-agent style tele with Lollars: low-wind Imperial PAF in the bridge and soap bar P-90 in the neck. Sounds like the cross between a P90 and a Strat neck pickup.

If you find this is out of phase with the bridge pickup, swap them.