How can I determine if File Vault decryption is really in progress?

There is no indication decryption has even started.

It now says that FV is on and the option to turn off is greyed out.

One reason for it to be greyed out would be if you did not click the "lock" icon to begin decryption, but that's obvious. I can think of no other explanation. Try it again?

All my Macs have been using FV for a long time with no ill effect, but you must be prepared at all times to restore a volume from an intact Time Machine or equivalent backup.

FV encryption itself conveys obvious advantages in that an encrypted volume is utterly useless to anyone without its key. But yours is potentially useless if you are unable to create a backup and if it were to fail—which may turn out to be the underlying cause, so be sure to continue TM backups.

The reason it failed may never be known. If macOS maintains a continual log of its encryption / decryption progress or lack thereof I have not been able to find it.