Think about it this way.
When you backup a file system the process looks something like this:
Discovery → Index → Backup.
For a file system , the discovery finds all the files and their metadata, writes their information to the index then backs it up.
For a vm backup, the discovery finds all the VMs’ and their metadata, writes their information to the index then backs it up.
In the case of the VM backup the index only has information about the VM’s, it doesn’t have a clue what it is in the VM.
So when you go to do a restore the process is as follows:
Browse the index for the data you want to restore→ Select the data you want to restore → Restore the data.
In the case of a VM restore the index you are browsing does not have any information about the file systems within the VM by default. What a live browse does is a pseudo restore of the volume that you look through, you are not browsing an index rather you are browsing the filesystem of the backup.
I hope that makes it clear.
The reason why I cannot answer definitively about agents, is because something like this or something analogous to this happens with any backup where you are not indexing the contents of the backup at the time of the backup.
For many of these agents there is the option to turn on post backup indexing but this has pros and cons.