I think depending on your device model you have to backup one or more system apps with Titanium:
- settings app (gearwheel)
- e.g. for Samsung devices also: SamsungOne and OneUI Home
but I haven't tried it, yet...
Maybe anybody else?
It works for sure with "Samsung Smart Switch", that I have tried and worked even on a Xiaomi-Android phone! You can even backup /restore with Smartswitch on /from a local storage/PC without Internet...
Titanium backup is never supposed to be used to restore system apps. Even the developer of TiBu says not to restore system apps with Titanium Backup. It will cause undesirable behaviors if you attempt to do so.