Secure folder is more like a second user account on windows, its a second space to download apps, keep files, that have no access to the local files on the other accounts. This means that it, too, will have apps such as Google play, in order to download more apps, aswell as background services related to any of those apps functionalities. This means if there are any apps in secure folder that need mobile data, they will use it. This includes Google Play just checking for updates.
If you don't need your secure folder apps to be constantly updated or provide notifications, and you only use it to store local files. Then you can disable all the apps inside, so they don't run in the background. You do all of this through the app itself, it has its own settings menu within it.
In comparison, i have no background data usage in my secure folder, despite doing alot of my work within it on different apps. So its easily achievable.
