Unwanted "Send as SMS" fallback for iMessage conversations using Messages for Mac
I use iMessage on my iPhone with the "Send as SMS" option turned off. (Settings > Messages > Send as SMS).
I have the "Text Message Forwarding" option turned on for my MacBook. (Settings > Messages > Text Message Forwarding).
Often (about once a day) when using Messages for Mac in an iMessage conversation the message will send as an SMS. Why? How can I disable this?
(I care because the number I'm messaging is an overseas number so I end up being charged about £1 per message.)
MacBook Pro 16″, macOS 10.15