Well after a short discussion with R&D, it may be that a very peculiar situation was found, and your particular situation has been marked as fixed for the next Service Pack for Adagio Receivables.

Without getting into it too much, the scenario was related to very specific amounts used for the payment and the invoice, and how calculations were rounding. It only happened when using certain values, which was why it was very hard to track down.

Look for this to be released fairly soon (probably within the month, but don't quote me!!)