The P-trap under my kitchen sink disintegrated the day after
Christmas. Simple DIY repair. No big deal.

I replaced the metal trap with a PVC trap, figuring it wouldn't rust
out or leak ever again...

I can't get the joint where the P-trap enters the copper drain stack
to stop leaking. It's leaking out around the sealing washer, NOT the
threads.

Tightened the nut. Still drips.

Replaced the PVC nut with a metal nut and rubber washer. Still drips.

Do I need to go back to a metal trap?