I copy pasted the emoji codes from Slack, perhaps it has something to do with pasting text rather than manually typing with individual keystrokes (don’t know if there is some JS listening for keyup events or something that may be missed with pasting from a soft keyboard).
It’s likely related to using a soft keyboard and not fully registering keystroke events or paste events.
I’ve since managed to get correct messages simply by using a desktop. I think a viable solution for now would be to disable the fancy but please add some CSS to give the not-fancy editor a monospaced font!