MPStoops

Hello all, this is my first post here but I'm not new to cycling by any means. I road raced CAT3 back in the '90s, MTB raced Sport Class back then as well, and have even spent some time on track at the Olympic Velodrome at CalState Dominquez Hills. Back in the day I always bought bare frames and built them up myself with the components I wanted - always Campagnolo on my road bikes and, maybe a bit grudgingly, Shimano on my mountain bikes. Heck, I even used to build my own wheels way back then, so I'm decent with a wrench. These days, I ride recreationally, for fun, and have gotten my girlfriend (a former marathon runner) into both road and MTB riding (she's crazy for MTBing!!!). So we picked up a Specialized Rockhopper for her last spring to get her going, and she loves the bike, but hated having the 2x8 Shimano Altus drivetrain. I upgraded her to SRAM Gripshifters at first (she's 4'11" and with small hands had trouble actuating the double push shifters so the Gripshifters have been amazing for her). Later on I did away with the 2x8 Altus and put on a SRAM NX 1x12 setup - new shifter, new DUB BB, new crankset, rear mech, chain, Sunrace MZ90 11-50T cassette (the only non SRAM bit in the mix) - and I've got a problem that I just can't solve. The bike skips every now and then on just the smallest cog. Sometimes under stress, sometimes just putting light power down. I've got a spacer on the drive side of the BB and a 6mm offset chainring. I've been researching this issue and know I'm not the only one dealing with it. Here's what I've tried thus far:

1. Adjust the high limit screw all over the place - from full out to so tight it won't even let me onto the smallest cog
2. Adjust the B limit screw from super tight to super loose
3. Adjust cable tension on the rear mech, from stupid loose (like, nothing shifts) to so tight it's laughable
4. Checked that the cassette lock ring is tight
5. Checked and adjusted the derailleur hanger for proper angle using an alignment tool
6. Pulled out the spacer on the drive side of the BB
7. Put a 3mm offset chainring on, with and without the BB spacer
8. Tried 30T and 32T chainrings, with and without the BB spacer
9. Adjusted the tension screw at the shift lever to the point I've almost worn it out (not really, but you get the idea)

Again, just to be clear, every part except the cassette is SRAM NX Eagle, including the chain. And, yes, it's definitely a 12 speed chain. No matter what I do, I get the exact same random skip in the drivetrain. I've got the same groupset on a Fuji hardtail, including a change to the same Sunrace cassette, and it never misses a beat. I've also got the same groupset on a brand new Specialized Stumpjumper, and it shifts great. But on her Rockhopper........it's driving me nuts. Everything else shifts beautifully, and when I set the high limit screw on the rear mech to keep it off the smallest cog, it doesn't skip ever. The chain line doesn't look odd at all, especially when compared to both my hardtail and Stumpy. I'm stuck.... what am I missing?