By default, when a standalone domain is added to a hosting package we reset the DNS records to the default 20i records.
This is to ensure normal services can function, including emails and the website itself.
In the event you have a standalone domain that has custom DNS that you wish to add to a hosting package, you'll need to be aware that the DNS will be reset when doing so to make way for the default DNS.
If you want to maintain the custom DNS you'll need to ensure it's added once the domain is assigned to the hosting package, so that those services continue to run as normal.