Should the tab bar be accessible on sub pages in a mobile app?

I've just joined a team as an UX / graphic designer developing an app for iOS and Android. The app is already in production, and I'm working on implementing new functionality and tweaking the current design.

Our app is tab based, and have five tabs for the main pages in the app. There are also several sub pages from each tab the user can navigate to.

One thing I noticed is that the tab bar is not visible / accessible when the user navigates deeper. Instead, you have to use the back button in the navigation bar to get back to the parent page before you can use the tab bar.

From my experience with other apps like Facebook, Messenger, Instagram, App Store, Spotify, SoundCloud and so on, is that the tab bar is always accessible from every sub page. This is something I think I'm gonna try to convince my team to do.

But still, what are the pros and cons with having the tab bar always accessible? Does anyone here have any experience with this issue?