Re: Shielding pickups...
That's a pretty powerful pickup for the neck position.
I don't think feedback is EVER a grounding or SHIELDING issue. It could be the sheer power you've got there. Strings have the most amplitude & output at the neck pos. - more than at the bridge. Neck pups should be lower in power, and bridge pups higher in power.
If lowering the pickup doesn't help, then it's not completely feedback, and may be squeal, which results from windings not tight enough, or not enough wax potting, which would be a rare, but probable manufacturing error.
I've been through this personally BTW with a similar pickups in Les Paul & had to send the neck pup back for replacement - problem solved.