virtualbox.org

Yes , it's an old issue indeed.
But , I have disabled the power saving function already.
So , the Vbox 6.1.14 works perfectly.
The guest died after I upgrade to V6.1.16.
Switch back to V6.1.14 , perfect again.

Yes , as you said the 6.1.16 is stable in linux guest ( I am using ubuntu )

I don't know why there are some issues under win7 and win10 guests.

Host: WIn10 / Guest : Win10

The guest will hang if you do nothing to the guest in around 10 minutes.

Host: WIn10 / Guest : Win7

The extra disk (2nd hard disk) in guest will be disappeared after several hours ...

When I change the version back to V6.1.14 , it works normally.