It's so much easier to diagnose stuff when it's right in front of your eyes.

I'd rule out a stiff link, because that would only happen once every 2-3 crank revolutions unless your chain was full of stiff links. It could be trim, or an upper limit keeping the RD from centering on that sprocket, but that would also mean that it would be hard to shift into it. So this is possible, but iffy.

Offhand, not seeing it, I suspect that the RD is too high and the upper pulley is too close to the sprocket. This could be confirmed by eye -- there should be 1/2" of daylight between the upper pulley and the sprocket, or 1-1/2" along the chain. You can confirm this without adjusting anything by shifting to the outer chainring. That will drop the upper pulley a bit, and if it doesn't skip anymore that would argue for this being the issue. You solve it with the B-screw adjustment.

If that isn't the issue, and/or if you've adjusted the limits, you need to reset them. Leave the lever in high hear, and shift the RD through the range by pulling the bare wire away from the frame like a bow string. Shift this way to the 2nd lowest gear, then to low. You want to back off the screw until you can almost cause the chain to overshift, then bring it back until you can still shift to low, but 100% bulletproof reliably not overshift. Test fresh by shifting to high, and shifting multiple sprockets to low. Retest 2nd low to low. Repeat the various attempts to overshift a number of times until you KNOW you can trust the adjustment to prevent overshifting. Then adjust shifting and trim bvia the barrel adjuster.