[O] Trouble with scheduled item with deadline on agenda

[Alex Branham]

Hello -

I have this item in one of my agenda files:

** TODO Pay estimated taxes
SCHEDULED: <2018-03-31 Sat> DEADLINE: <2018-04-17 Tue -20d>

and org-agenda-skip-deadline-prewarning-if-scheduled set to 'prescheduled. 
Since it is scheduled for a date later than today I would expect that it 
doesn't appear on my agenda today. However, it looks like the prewarning -20d 
is causing it to appear anyway.

Am I misunderstanding something here or is this a bug?

Thanks,
Alex