An update:

I’m not out of the woods yet. I went to the range with two boxes of intermediate-power ammo (240 gr SWC, 18.0 grains of 2400) and one box of full power ammo (240 gr JHP, 24.0 grains of 296)

I started off with the SWC and had one misfire in the first chamber—the sixth round failed to fire as the cylinder had unlocked and rotated backwards one. But I then fired about 70 more rounds with no problems.

I then switched to the full power rounds and had two failures in the first six rounds. When the fourth round fired it unlocked and rotated backwards one, and the same thing happened when the fifth round fired.

I packed up and brought it back to the gunsmith for further troubleshooting.