I was able to test and proof this works fine when user is deleted form WP backend. Therefore I opened a support ticket at Ultimate Member. They are working on it.

Thank you. It is very hard to deactivate my membership plugin (Ultimate Member). I deleted the user account from the account page of Ultimate Member. Although the user is not visible at WPForo frontend (replaced by Anonymous) and the user is deleted from WordPress database, it still exist in the WPForo profile table. Don't you think this needs to be solved between Ultimate Member and WPForo?

As an alternative I can write an action hook. Do you have documented functions to delete a user from WPForo?

It seems you're using some other profile plugin, that has an effect on this process. I mean that doesn't allow to delete users from avatar. 

To make sure just deactivate other plugins and check again. 

I found that deleted users still exists in the database and are counted as member in the usergroups. Deleting the records in the database solved the member count issue.