Answer
This is a side effect of also setting LineItemNo in the payload: passing any LineItemNo greater than 0 switches Order_Quotation_Line_API's internal logic onto a different path that effectively discards or nulls out some of the other values you passed, including ProbabilityToWin, before the mandatory-field check runs. Omitting LineItemNo from the payload (and letting IFS assign it) avoids the problem and lets the API behave as documented.