I’m not sure of the exact timeline, but I think I started having intermittent rear seal leak issues when the gasket kits stopped using the hard white-ish side seals for the retainer.
Those were the ones you dipped in mineral spirits, and just slipped them into place.... then after some time they would swell up in place to fill the void, after the seal cap was already in place and torqued down.

Once those went away, that’s when the problems started to pop up for me.

This thread is a good tutorial for someone who’s had a chronic leaker.