For some, not all, contacts in Messages – who I know for certain are on iPhones – when I send a message from macOS the message is first blue, then green, then blue, and the sticks to green. No big deal in the sense that the messages always go through, but I’m curious why this happens.
I’ve seen green bubbles for those I know are on iPhones when I’m in an area with poor data coverage, the same if the recipient is in an area with poor data coverage.
I’ve also seen weird behaviour when someone has switched from iPhone to Android, but that tends to be a 1 time thing.