Exactly, I'd recommend .010" next because if you give matching tapers too much of a running start, they have that much more room to get really crooked. Another slight powder increase might be worth a try, too, in hopes of closing up the vertical or at least seeing if the vertical is powder-related, or something else.
When Fiver gives advice, there's always a good reason. We regularly see the need for about 5% more powder for same velocity when adding powder coat. 5% was a best guess to put your load back on the same accuracy node you had worked out with the uncoated bullets; the balance of the accuracy workup amounting to bullet fit, case prep, alloy, etc. I think you may need just a touch more powder yet, and one more thing......a neck tension check: Pay close attentoon to how the press handle feels when seating the bullets and sort into piles the ones that feel light or heavy. You saw how much your groups changed by changing seating depth just a little. Well, when you introduce bullet jump, suddenly neck tension begins to matter, particularly the consistency of it from one cartridge to the next but also the amount. Add powder coating and neck tension suddenly matters even more because the slippery coating affects engraving resistance and thus powder burn consistency...and groups.
Now that you've made some changes and noted the effects, is what we've said starting to make sense?