The instructions are unclear on how to connect the rotary encoder. If you look at the encoder from the rear (shaft facing away from you), with two terminals on the left and three on the right, the correct way to wire it is with the black wire on the top terminal and the brown wire on the bottom. The diagram and the written instructions show those two wires reversed, but the photograph in the instructions shows them connected correctly.
I guessed (incorrectly) that the way in which two pieces of information indicated was the correct way was the way to wire them. I also guessed (correctly) that since the two wires on the other side were for a normally open pushbutton switch, and the middle contact on the encoder side was grounded, the controller was simply detecting which of the brown and black wires were getting grounded in which order to determine the direction of the shaft rotation, so there wouldn’t be any drastic consequences to wiring those two wrong, just reversed tuning.
Naturally I opened it up again, warmed up my soldering iron again, and switched the two wires around. Every normal radio in the known universe has a tuning knob where turning it clockwise makes the frequency go up, and it was very annoying to have a radio that acted in the opposite way.