I suspect this is a bug and should be treated as one. However in case I am missing something.
Put a bid on Hamilton, max of 300K points (all I had at the time). the bid showed 250K which was above the next highest bid.
When I logged into moments.mr, it showed my current balance as the now 330K current MR total minus the current bid, or 80K. Makes sense.
When I tried to increase the max bid, it told me I didn't have enough (because it thought my total was 80K). Basically it is counting the current bid twice.
Didn't matter as it closed at 255K but still is an issue anyway.