Change the Time Format from Short (which I guess is the default) to Long, as shown in the screenshot above. Then the reminder will include the seconds portion of the alert time and will give you the full duration, not some truncated amount.
I’m surprised at this behavior, as I had assumed that the date/time formats mattered only when you were going to insert it into a text field or string. Setting a reminder date, I thought, would use the underlying date/time numeric value. Live and learn.