Ah—yes, after some further fiddling I realized what was happening. It’s the flywheel/pilot bearing. The bore in the flywheel is too tight, and I just pressed the bearing in like a monkey. It spins the input shaft because there’s a lot of bearing drag, particularly when it’s cold. When the flywheel warms up, the bore increases, and the load on the bearing decreases, and puts less torque on the shaft, but it’s still a problem.

Its minor tho, so I just won’t do anything about it. If I ever do the clutch again, I’ll make sure the pilot bearing spins freely!