Thanks @agsteele, but that's not the behavior I see.
Click (+), then Task, a new Task is created. I enter text as the task description, click on the calendar and select, say, March 30, then click "Done". Then I click on the "Bell+" icon to set a reminder, and the default date is as you said, within an hour of the current date and time. I would expect the default reminder time to be the due date, not the current(-ish) date.
I think the difference is that you're setting a due time for the task, as well as the date, and I'm not. When I add a time to the due date, I then see the reminder options relative to the due date.
I would argue I shouldn't need to specify a due time (which is generally meaningless six months in advance), if I want my reminders to be relative to the due date.