I was right!
Bug confirmed, and fixed.
The fix will cause a slight downward adjustment in manpower gains. Not much, about 1-4 less for some countries, at their current recruitment levels.
Example: Before the fix Japan had 1021 own manpower, and 402 for other sources. After the fix it is 1017 and 399.
This change is due to the way things are done when calculating in ATG. If you divide something by 10, it rounds down to nearest whole number. So the remainders are not considered. So if you want to multiply something by 1.3 you would have to do it by multiplying first with 13, and then dividing by 10. I had to switch some of the dividing and multiplying to a different order, thus there are some slight rounding issues. Not big stuff, but still, thought I'd mention it.
The problem was that a certain amount of digits is reserved for the size of a number. If calculations bring that number above the digits allowed, the computer interprets it as there is a minus sign ahead of the number instead. I was hitting that number.
I'll put it into the next version I get uploaded (1.0.2)