Hello @Burner!
I just double checked your setup using all numbers fields and the formula returns as expected:
So I’m not sure why exactly this is happening, but it might be worth it to double check that the type returned for both your lookup and rollup fields are formatted as an integer:

If that doesn’t work I’d recommend contacting customer support here, because this might be a bug. Sorry for the frustration here and let me know if I can help with anything else!
Hello @Burner!
I just double checked your setup using all numbers fields and the formula returns as expected:
So I’m not sure why exactly this is happening, but it might be worth it to double check that the type returned for both your lookup and rollup fields are formatted as an integer:

If that doesn’t work I’d recommend contacting customer support here, because this might be a bug. Sorry for the frustration here and let me know if I can help with anything else!
@Burner - my guess here is that one of your numbers has some fractional part to it that isn’t displayed in the UI, so they all look like integers but one of them isn’t, giving rise to a negative number, e.g.:
-0.0034564
which is then displayed as:
-0
It could be a correct calculation of your underlying data or it could be a bug somewhere