Every order comes in at one and the same amount
The conversion settings hold one fixed number, and it is that number that goes to Google, not the order amount. Revenue in the report is simply the number of orders multiplied by it. ROAS computed on that means nothing.
How the portal sees it
The portal compares that number with what actually arrives. And it catches the second case: the settings say “pass the real amount”, while in fact every order still carries the same single number.