People can do things without meaning to, but when someone brings you cupcakes knowing what they do, then yes you have to ask some questions - it sounds very passive aggressive to me.
The main thing is to be assertive - I recently had an aquaintance throw away a dozen cupcakes her hubby brought in and he said "why did you do that?!" and she said "because I told you I would". he had bringing "treats" in for himself - not the two or three cupcakes he would have ate, and not putting them somewhere she would not need to come across - but a dozen or more, and on the counter near her foods. So she stepped up and threw them out - opened the box and dumped them into the trash.
We can't compromise on ourselves, and we cannot allow others to do so either. Nope. Nada. shhhh. don't argue.
Good luck.