Xbox one losing saves

Hi there. My husband is playing on Xbox one X. He loves the game, however hes having problems with the game losing his save files.

His first play through was 16 hours and when he turned his xbox on the next day the option to load the game had disappeared.

He was annoyed but carried on. His next play through was 4 days. Then when he turned his xbox on again, the same thing happened and hes had to start all over again

To say this is annoying and frustrating would be putting it mildly. Is there a known way of making backup saves or retrieving this info or is it gone forever?

Many thanks

Have you tried saving to the Xbox live Cloud? Or If you do: have you tried Switching Cloud saving Off?

Thank you Dacrimal
Hes tried both manual saves and automatic saves and its ok for a few days then it doesnt give him the option to load a game only new game. No idea why.

Try backing up your saves to the Cloud or an external Drive, that way, you can restore them if they disappear.

His cloud is switched on and he has an external hdd. Hes checked other games and they all sync when he accesses them but its like the save goes are being wiped from the game screen.