Distribution of genotypes after adverse selection
If you distribute the 0.12 equally to the two other genotypes you change the relative frequencies. You have to withdraw 0.12 to 0.48 and then set the three frequencies so that they sum up to 1. Therefore, the correct calculation is the following:

If we set $fAA = 0.36$, $fAa = 0.48$ and $faa = 0.16$ and then simulate the death of 25% of the Aa individuals, the same variables at the next time step (without reproduction) will be...

$$faa = \frac{0.16}{0.36 + 0.36 + 0.16} ≈ 0.1818$$

$$fAa = \frac{0.36}{0.36 + 0.36 + 0.16} ≈ 0.4091$$

$$fAA = \frac{0.36}{0.36 + 0.36 + 0.16} ≈ 0.4091$$