I think that Baan doesn't consider the supplier at all when the maximum order quantity is exceeded and puts the whole quantity to the other supplier who doesn't have maximum order quantity restriction. This assumption may be applied to case 2 and case 3. For case 4, does the supplier code of S2 precede S1? I assume the priority of both the suppliers are the same. Is it so?
The functionality you want can be achieved using the 'supplier capacity', capacity tolerance etc. in the same session on the last form.