Just had that identical situation on a 2016 F150 on Monday. The customer said he noticed the problem after the truck sat idling for several hours at a fire scene (he's a volunteer fire fighter). I can't help with any explanation as to why this happens, but if memory serves correctly, I think we've seen other Ford's do the same thing in the past.

James Kuhns
Owner/Technician
Auto Tech Solutions
Chatham, New York, USA