Is a thing with Ajax calls (i.e. fetching info without refreshing the page) that gets rid of all characters with a sufficiently high Unicode codepoint (every character has its own number, majority of emoji have very high numbers).
Preview and quick edit use that, so those will kill emoji, but regular posting and editing doesn't.
Is indeed a bug as 22 mentioned. SMF is fairly old so it doesn't surprise me on that department, but in this day and age it is strange they didn't fix it yet.
Haven't bothered to look into why it happens exactly. SMF has more funny problems.