At the time of a PO line creation, the trade agreement price in effect is inserted into the record line. If the users changes that value then or later, no validation of that change is done and no warning is offered that this price change is in violation of the trade agreement.

When I read about the trade agreement evaluation requirements that are part of the Setup, I had assumed that for a manual line or any other line or change, the line is evaluated back to the trade agreement. This evaluation is only done at time of line creation and if the values are overridden at creation or at a later date, there is no further evaluation or validation that the rules of the trade agreement continue to be enforced.

For instance, when I create a line with a trade agreement value, that value/price is populated in the PO line and I can change it prior to saving the line. That to me would not meet the evaluation criteria as the saved price is not being referenced back to the trade agreement for correctness.
Or if I update a previously saved PO line with a different price to the trade agreement, that change is not validated and evaluated back to the trade agreement.

To summarize my expectation: the system will suggest a price based on the trade agreement in effect at time of order creation. That is the last time that the trade agreement is referenced. If the price, quantity or dates are changed later, there is no validation back to the trade agreements for that item and vendor. This validation should be done and the user notified of changes that do not alighn with the in effect trade agreement.