Brifter won't downshift

I’ve got a bike with a Dura Ace R9100 drivetrain. About 800 miles on it. Until my most recent ride, no problems.

On my last ride, the rear derailleur would occasionally upshift spontaneously, and the drivetrain sound was a bit noisy. I assume that I needed to tweak the barrel adjuster.

I’ve got my bike on the stand right now for a cleaning and to make that adjustment. I removed the rear wheel, upshifted so the derailleur would land on the smallest sprocket when I remounted the wheel, and then remounted the wheel. Now the rear derailleur will not downshift–it feels like if I push on it any harder, something will break.

I can get some slack in the cable at the bottom and move it in its housing, so I don’t think the cable is stuck, and subjectively, it feels like the sticking point is in the lever mechanism.