Since we do not have access to the Mobile Leaderboards on PC, could you...
1). Look at some of the names on the leaderboard
2). Go to their profile and check their Score History to see if their scores are being posted?
EDIT: I signed into PCEA. I was able to see the Mobile Unlimited Leaderboard. However, the option to change views for Tiers is not in working order. I could only see the leaders in the Champion Tier.