I was having same issues with Find My Phone with W10M on L928 yesterday.
I just checked my Windows Insider and found that I was not in any Ring for some reason (guessing after the last Windows Insider update).
I put my L928 back into Fast Ring and rebooted.
I can now use and see my L928 on
https://account.microsoft.com/devices AND the date and time line up exactly to when I rebooted AFTER putting it back into Fast Ring.
Not sure if it has to do with the device reboot and/or being in a RING or not.
I will follow up tomorrow and check it out again and see if it stays available on Find My Phone or not.
EDIT: And I just tested RING and LOCK from the website and both work as designed. can not test ERASE as I dont care to loose all my data at this time.