The problem seems to be fixed. Maybe I just had to restart my browser?
Clicking on a Chord Preview button (such as the ii chord) will not sound the same as the ii chord that is part of the puzzle. It said I was wrong when I dragged a chord (in my case the V chord) into the rectangle that played the V chord. The preview sounded exactly the same as the “question” chord, and as far as I know there was no extension that could be fooling me. In the following puzzle I tried, none of the chord previews matched what the chord question was.
This was in the “Crush” mode. The instrument was set to “Acoustic” for one puzzle, and I changed it to “Piano” for the next.