yeah, I remember discussing with one of Enscape team members why 'safe frame' isn't automatically shown in preview even after it's checked on (it could be just opaque not just black if the team doesn't completely want to block the leftover view). At least there was a way to keep the safe frame in preview before, now there's no way. I guess not many people care about 'safe frame'?
I can suggest you to install a sketchup safe frame plugin. İt works very well with enscape.