In my opinion and experience the system very rarely “messes up”. There’s always a chance… and if so, you’d need to contact customer support and prove that you had more. Even then they can apparently see when and where each crown was spent.

My guess is a couple things:
1) maybe it just slipped your mind and you spent the crowns before you went on hiatus.
2) you were a player that abused/exploited crowns and they were taken (highly unlikely, since you’d probably be banned versus just stripped of crowns).
3) you had a subscription that gave crowns but you cancelled or refunded.
4) someone had access to your account and gifted themselves items.

I would venture to say #1 is highly favorable to be the most likely option as we’ve all been there before in life. If you truly believe something happened erroneously, I’d do your best to search for old screenshots or videos and then reach out to customer service to see if they can see if your account had been accessed during your hiatus.