Hi, well normally this function in virtualmin is designed to be like that and I even encourage this to use it - however you can make sure only your ip address can access this part - if you on same lan, just allow any ip from lan if you are not you can setup something like openvpn to access it… it depends of your needs. What I used to do it was locked down pma with htaccess file to my lan IP but nova days I use ssl client cert. Basically you apply htaccess to redirect and allow only https and use your client cert to authenticate = if no valid cert found on your device - you will have an error page no matter what you do.
Another way is to install pma on your system manually, just download it, do your salt/hash settings and you are done. If that is outside of domain like your private folder, let apache know where it is sitting - I can help you with that if you need it but there is plenty articles how to do this. In anyway you decide, I am free for paid consultation regards custom install of pma into your system.
have nice day.