What possible explanation could there be that Bitcoin Core on Linux Mint claims to but does not save the wallet backups?
What possible explanation could there be that Bitcoin Core on Linux Mint claims to but does not save the wallet backups?

Possible explanations include:

The path to the saved file is not what you think it is

using a relative path rather than an absolute path
using a chrooted program


Some higher level of security is preventing you seeing the files

SELinux?


There is a bug in Bitcoin core. This seems unlikely but if you can reproduce the behaviour on another machine you could document the exact circumstances in a bug report.


footnote 1
All normal Linux systems have a large number of default users and not just the one you use to log in normally. If you cat /etc/passwd you'll see a list. It will include a root user (and around a dozen others) unless the system has been substantially modified from the typical arrangement.

footnote 2
You can make your own backups of the wallet.dat but, for extra reassurance, you could practice extracting the private key using the debug-console then importing the data into a new wallet (of any type) on another computer and verifying that you can create spending transactions there.