If any-one else has this happen to them I thought I'd share that I've found the solution.
I stopped getting the trip updates to the app "out the blue" (pardon the pun) last year around October; happened with both the new app and the old one.
Yesterday I used my nav and located a destination via coordinates - up popped a message saying something paraphrased "this doesn't work unless you do something with your bluelink settings" - I didn't really read it, just followed the link to my settings.
I didn't "do anything" on the page put pressed ok, and all of a sudden after that journey, my app had the correct trip mileage and the journey had sync'd.
It appears I'd been silently signed out - without actually being signed out on the car (as all credential were entered etc).