I have 7.1, and just went out to emulate your observation. For five minutes, I could not duplicate it...until I altered, in "Settings", some of the vehicle's Auto-lock options.
There is child-lock, driver-only or all-door, lock on walkaway, etc., as I'm sure all of you are aware.
WE keep all of these options in the miminum security set-up. It was only when I altered them to some more secure possibility that the Trunk Remains Locked gray screen the OP mentions came into being, so it appears it's a feature of that.