"Hey Siri" default to speakerphone stopped working

Afternoon community,

Out of the blue the brilliant new feature of defaulting calls made through the "Hey Siri" command to speakerphone has stopped working.

I can still get it to work if I say "...on speakerphone" but it is no longer doing it automatically.

I have looked around the forums for a solution but thus far, nobody has the answer. This isn't ideal while I am driving!

Can anyone help me out?

Kind regards,

Adam