MacBook Pro M1 - Time Machine not backing up automatically

Anyone else with the new MacBook Pro with the M1 Chip having this same problem?

I have just got a new MacBook Pro with the M1 Chip and I'm using Time Machine for backups to a NAS and also to a portable USB HDD (it alternates the backup destination). I have found that whilst I can start a manual backup, the hourly automatic backups do not happen unless the MBPro is connected to its ac power adapter.

My wife's new MacBook Air also with the M1 Chip does not suffer this problem, it will start a backup every hour to the NAS (or to the portable USB drive if it's connected), without being connected to its ac power adapter.

I have also noticed a difference in the Battery Menus within System Preferences

The MB Pro has an option to "Put hard disks to sleep when possible" the MB Air does not have this option.

So both these MBs are the new 2020 models with the M1 Chip, the MB Pro has 16GB RAM and 2TB of SSD, the Air has 8GB RAM and 512GB SSD, Both are running MacOS Big Sur 11.1

I cannot understand why the MB Pro is not able to backup automatically on battery power but the MB Air can do this and I have not been able to find a menu setting in either MB that allows me to choose to backup on battery power.

Also, I'm baffled as to why would the MB Air not have the option to put the hard disk to sleep whilst the MB Pro does have that option?

I'd be interested to hear from anyone else who has a new M1 Chip MacBook Pro if they are experiencing the same issue of not being able to backup using Time Machine unless connected to ac power. Does anyone with a new MacBook Air with the M1 Chip have the same missing menu item for putting the hard disks to sleep in System Preferences > Battery?

Thanks

Adrian