I would bet on either pickups not potted well enough, or tubes going bad or incorrectly supported against vibrations.
You've stated pickups were potted not too long ago, so it would stand to reason this might not be the reason.
Romulan Logic dictates to me that if only one guitar has this feedback issue, that guitar is likely having a problem.
If feedback occurs with other guitars, it must be the gear.
Checking and replacing tubes is inherently easier than re-re-potting pickups.
So, start checking whether only one or all guitars creates feedback, then check/replace tubes - especially long-plate tubes, which from comments on the web are the most prone to feedback.
I don't think cables will cause such, but c'mon, you have tried different cables, haven't you?
Joke: Tried the old telephone smash trick?
Lift up the telephone and smack it hard onto the table's edge.
If it still works, it's a good phone.
If the squeal is gone, it was the phone, so maybe go buy a new one before the squeal returns.
If the squeal is still there it is indeed the phone, so go buy a new one.
If it doesn't work after this, and the squeel is gone, it was likely the phone, so go buy a new one..