Same issue; I gave all the answers on the website at once (already knowing them, since they were the riddles from the book). When I logged into the mobile app I received notifications up to the fourth riddle and the final riddle. I don't have the cards to complete the final task.

I have the same issue. I got the first, second, third, and fourth riddle answers. I received an in-game mail for the fifth riddle and put in the answer. But nothing has happened in-game. I thought it was broken and put in the answer for the sixth riddle (Fish) and received the reward for that. I have the seventh riddle in my inbox, but I'm not getting credit for the answer (Time).

Kind of a bummer that the riddle thing is broken and some people won't receive the rewards for it.

Mine did the same thing, then I realized I'd put in the wrong answer for riddle #4 (it was actually the answer for #7 and gave me the rewards for riddle #7, so I didn't immediately realize it was wrong). Once I put in the correct answer, it gave me riddle #5 and so on.

I have the exact same issue. It skipped from four to eight.