Messages showing green rather than blue since ios 18.3 and Sequoia 15.3 upgrades

I am experiencing strange behaviour on the Messages app since upgrading my Macbook to Sequoia 15.3 and my iPhone 13 to ios 18.3. A breakdown of what is happening:

• Messages to some contacts are fine - receive messages in blue, send messages in blue (fine, working as before)

• Messages that I am sending to some contacts which were previously showing in blue are now showing in green (issue A)

• Sending messages from my Macbook shows the same issues - apart from I also can no longer send any messages to myself (show as 'Not delivered'). (issue B)

When I talk about showing in green / showing in blue I am talking about the background colour of the message in the messages app, which I believe shows blue when messaging another iphone / apple id, and green when not.

I could not find any info on issues A and B when searching just now, could anybody help with this please?