I had the same problem got 3 stars with the trx i only got the guide line but nothing else.
I fix it i changed my hud to show notifications and went to solo mode.
Not sure which one solved the issue but i did get the pop up about the riddle being solved.
I hope this helps you.
I have the same problem, but only with the second treasure. I tried your solutions, but so far nothing has helped 
I had the same problem with treasure 4. Changing cars solved it for me.
I’m having trouble with treasure chest for 2 and 3. Did the tasks in the cars required got one star on one and 2 on the other and they both did not complete.
Found the solution! I had the same exact problem. I had my “notifications” tab turn of in the hud and gameplay settings. I then turn it on, drove thru the speedzone and it work straight away.
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I have done the treasure #1 but when I completed it and smashed the chest it never unlocked the riddle for the treasure #2. I have tried to do the needle descent but nothing seems to be working. How do I unlock #2.
Treasure challenges unlocked for me, by playing the game - completing races and PR stunts.