EDIT : solved by going to About in Settings and changing device name from 'MIBOX3' to 'TV'.
Now I can say "turn off the TV".
Note that saying "turn off MIBOX 'three' " did not work as confirmed in the thread.
(NB, I'm french : it's not TV but "tele", but my guess is that it would work the same)
Now google assistant answers it does not find "tv" device... it seems the assistant is now looking for a google home device and not basically sending a CEC command to switch off my connected tv...
Found the issue on a MIBOX3 ...
Same issue here - MIBOXS