Sounds like the C/A remote opens only driver's door, in which case tailgate can't be opened from outside (expected, tailgate and 3 other doors are locked).
There's a setting in iDrive to choose if first click of "open" button on remote opens all 4 doors (plus tailgate) or just driver's door.
I am guessing comfort access remote sends an "open" command, then iDrive interprets what doors (just drivers vs. all 4 + tailgate) to open.