Solved! Go to Solution.
I found a solution, though it is tedious to setup, but works once you have done.
Let's think about Secure Folder as being a separate lock screen. So, our phone has 2 lock screens.
Once you are are in Secure folder, go to settings (by clicking on 3 dots) -> Notifications and Data -> and enable the "Show Content" and "Hide Content on Lock Screen" options.
And now begins the tedious work.
Start, by going in Secure Folder Settings -> Apps -> <App Name> -> Notifications. In this you will see various types of notifications possible in the app. Open each one, change "Show Content" to "Hide Content" under "Lock Screen" option. Basically what I feel is that, Samsung by default as left all the notification settings on "Show Content" which makes it show outside, or if you change it, it doesn't show anywhere (even within Secure Folder).
I checked this solution by sending email from one account to an other email account logged in Secure Folder Gmail, and by playing with "Show Content" under individual App settings, you can have more control over the things which you want to show.
If this works, please like and promote this as a solution. It will hopefully get attention help Samsung to implement in the upcoming patch.