Recalculate Amount on P4312

When I change the quantity on a purchase order, it does not recalculate the extended amount and throws the error "extended price invalid".

On what actions is the logic to recalculate triggered?

If there are 2 lines it is not an issue as when I move to the next line, it is recalculated.

The problem occurs if there is only 1 line, I cannot move to a next line since there are none.