Conversation with contact always sends as iMessage

When I send a message in a conversation, the send button is always blue even when I have no internet connectivity, the green send button is never available.

This issue affects only two contacts on my iPhone.

This has been an issue for months now, I have tried the following :-

- Disable iMessage, reboot iPhone, enable iMessage

- Delete and re-create contact

- Split mobile and email address between two separate contacts

- "Erase All Content and Settings" completed on phone, setup iPhone with same iCloud credentials

- "Erase All Content and Settings" completed on phone, setup iPhone with new iCloud credentials

The only way to send a text message is to double tap the iMessage that has just been sent and selecting "Send as Text Message".

Any suggestions or resolutions would be really appreciated.