Why is my iMessage sending as SMS to just one contact?

Hello, just one contact of mine seems to be affected whereby I send an iMessage but it gets sent by SMS. They have an iPhone. Worked before I went away on holiday…and it literally is just this one contact that I have a problem with.

They have iMessage switched on they say.

Any help? Not the end of the world…but it’s bugging me and my OCD😬

[Re-Titled by Moderator]