Hi,
System behavior seems correct to me. First you posted 0.991 qty with amount of 2000. Now you increased price from 2000 to 4000, but that is for the balance qty, that is, 1 - 0.991 = 0.009. You increased the price in PO from 2000 to 4000 but your qty is fixed ( 1 AU) out of which 0.991 is already posted. So, though the price is increased, qty is fixed. So, system proposing the balance qty 0.009 and which is correct.
As per your analysis, system should propose 0.505 as balance qty then the sum of the total qty will be 0.991 + 0.505 = 1.496 but your PO qty is 1 AU. So, proposing 0.505 as balance qty would be incorrect.