I doubt it is actually deleted but sent to the chest (though for most email programs it can't be inserted into the email database file, effectively deleting it) Check the chest and see if that what was done in the auto actions.
You can change the automated actions from first action move to chest, to Ask that way you should be able to allow it through.
From the avast UI, Real-time Shields, Mail Shield, Expert Settings, Actions and change the automated actions, see image.
Why does it delete it (as you say), what reason is given ?