Ref backup-domain – Virtualmin
In the documentation its say to use --feature and the feature name.
--all-features option will be used to include all virtual server features in the backup, but you can instead use the
--feature option one or more times to control exactly what gets included. In this case, it is wise to use at least
--feature dir to include each server’s home directory.”
But its a guessing game to work out what the feature name is, can you add the documentation the feature names and/or using the help command get a listing of the feature names.
i.e. virtualmin backup-domain --feature help