Why are my 350 crowns disappeared without getting a reward?

Did you trade it for an album pack in the crown center? If so, you'll need to go back out to the main map to get the reward. If you still don't, I would contact support.

And always screenshot before you use the crown center. Last album event, I had enough extra to get 200+ wild cards. When it ended up not working, support said they don't keep logs or have any way to verify I had that many crowns, so they gave me 8 free games... which was about 20,000 coins.