Titanium Backup confirmed on Twitter today that the development team is working on adding support for Adoptable Storage to their app. This is going to allow users that utilize Titanium Backup for their backup needs, to be able to also backup items that are stored on Adoptable Storage within their smartphone or tablet. The reason why it hasn't been done already is due to the fact that Adoptable Storage is a bit strange, and it's a feature that not every smartphone has. Basically, Adoptable Storage takes your micro SD card and formats it, then basically fuses it with the internal storage on your device. Allowing you to have more storage, without paying a ton of cash. But users are still able to remove the micro SD card, which makes Adoptable Storage a less-than perfect option, and also means that not every app or game will be compatible with it.
Those that like to keep their devices backed up, in case something does happen to their data, have likely used Titanium Backup at some point. This app is one of the most powerful backup solutions on the market, and for good reason. It used to only be available for rooted devices, but it does work on non-rooted smartphones and tablets as well now. Titanium Backup can backup just about everything on your device, including the OS, which is especially good for those that like to flash ROMs a lot on their smartphone. Titanium Backup is a free app, but some features are behind a Pro key, which will cost you $5.99. For the amount of features you get from this, the price for the Pro key is more than worth it.
Titanium Backup did not mention when they will support Adoptable Storage, but noted that it is being worked on, so it should be coming soon. Since Adoptable Storage launched with Android 6.0 Marshmallow, they have likely been working on it for quite some time, seeing as Android Marshmallow has been out for over a year already - it launched in the fall of 2015. Hopefully this feature will be available sooner rather than later.