High Sierra can't mount partially encrypted apfs...

I decided to encrypt my archive (2nd unit, 3TB HDD), so I started Encode.

After a few minutes, the volume reappears and the encoding continues in the background.

After about 2 hours I need to turn off. The encoding was proceeding but pc go off without any warning.

When I start up again, HighSierra requires the passphrase (2 times), but it can not mount the partially encrypted volume again, and not continue encoding

My drive (HDD) was formatted in HFS +, why it is now formatted in APFS?

In the previous operating system it was possible to shut down and restart, encryption proceeded later on.

Please, help me.