The game can only reduce the bucks count with input from your account; it cannot do so independently. So, if the bucks go missing, unfortunately, something that happened on your account is the culprit. 

Possible Causes

One explanation may be that you're in the Edit House mode, utilising the "Undo" or "Clone" buttons to purchase furniture. When you sell an item in your house that was added in another session, you receive a reduced amount of bucks. If you attempt to undo what you had sold, you will purchase it at its total price and find you have lost bucks. We would suggest that you only sell furniture when necessary. 

You may have enabled an auto-clicker that clicked and purchased an item you didn't intend it to. We advise against the use of auto-clickers, and you can see our article on this here. Our team will not restore bucks lost due to the use of an auto-clicker. 

It is possible that your account was hacked. If so, please review our guidance on what to do when your account is hacked. This is evident when you see that your bucks have been spent on plenty of items which have been traded away. 

The absence of your bucks may have been caused by potential data loss. If you played for a certain amount of time, left the game, and re-joined to find the bucks you earned during the last 5 minutes of your play session had disappeared, you may have experienced your progress not saving. If you believe this is the case, you should report it to our team at https://www.playadopt.me/support as a bug.