After a bit more investigation, I've found what the problem is: dueAfter works correctly if you specify only a date, but not if you specify both a date and a time. For example, assuming today is Saturday:
Task 1, scheduled for Saturday @ 4pm is captured in the list(incorrect)
Task 2, scheduled for (plain)Saturday is excluded from the list(correct).

I find this a bit strange, as I thought the function would only check the date and ignore everything else?
Is there any way around this?

I've also stumbled upon this, thinking that date+time works as expected while only date is trickier. Depending on the direction (dueAfter/dueBefore) today is either 00:00 or 24:00 today, if that makes it any clearer ;-)