Dear User,
Great question! The reason you are slightly off in your calculation is because the Malawi DHS 2015-16 is one of the first countries to start including the day of birth into the age calculation of the child. You can read much more about the change in calculation of child's age in the link below.
https://dhsprogram.com/data/calculating-the-age-of-children. cfm

So instead of SELECT IF (AgeMonths < 24) you need to use SELECT IF (B19$1 < 24). I have included my code below

*// Weighting table.
COMPUTE rweight = V005 / 1000000.
WEIGHT BY rweight.

*// Selecting only the last child 0-23 months old.
SELECT IF ( B19$01 <24).

*// Percentage who took 2+ doses of SP/Fansidar and received at least one during an  ANC visit  .
IF ( M49A$1=1 & ML1$1>= 2 & ML1$1<= 97 & ML2$1 = 1) SPANC2 = 1.
VARIABLE LABELS SPANC2  "Percentage who took 2+ doses of SP/Fansidar and received at least one during an  ANC visit".

FREQ SPANC2.