Download the logs, and see if the mileage is ticking up there -- the MBB's view of the mileage is on every periodic status line in km. The MBB and ODO keep the mileage independently, and the MBB syncs them both to the highest of the two regularly. If this sync happens, there will also be a notice in the log. I had the opposite problem where the odo got stuck at 2^31km, but here is what the representative log lines look like when it happened to me. Zero replaced my dash and had the dealer reset the odo in both the dash and mbb to get back to normal.
07907 08/27/2016 21:31:00 Disarmed PackTemp: h 37C, l 35C, PackSOC: 15%, Vpack:98.236V, MotAmps: 0, BattAmps: 1, Mods: 11, MotTemp: 36C, CtrlTemp: 30C, AmbTemp: 32C, MotRPM: 0, Odo: 2894km
07908 08/27/2016 21:31:00 Batt Dischg Cur Limited 414 A (60.0%), MinCell: 3499mV, MaxPackTemp: 37C
07909 08/27/2016 21:31:08 b'Dash ODO 2147483648km > MBB ODO 2894km. Setting MBB to 2147483648km'