I'm finding the leaderboards frustrating on the PS4 too.
No friends list and times you've set sometimes don't get logged.
Yes, leaderboard times are only set in TT mode and no, they are not cross platform. Practice/free run/race times don't count.It is very strict about track boundaries in certain places, but not others, a box does always pop up when you do return to the track if you've gone too far and the time won't be accepted.
I assume all leaderboard times are set in time trial mode? are they cross platform.
Actually I have quite a few time trial questions, time to start a thread of it's own me thinks.