Thanks for the problem report, afatcat!
Do you have the game number of that game or do you know if it was a rated game?
I just tested the situation and couldn't reproduce it. What's even more strange is that the game shouldn't allow the second person to even bid less (or an equal amount). Normally, when the first player bids say 6 debt, the other is given the option to bid 7 debt or more, see the screenshot from my test game.
Was anything else 'unusual' going on, maybe an extra turn or something like that?