When I use my fancy Compose key to enter 😀 (or via the Win+. emoji panel), I just get this on stdout:
invalid code point 0xd83d
invalid code point 0xde15
Not sure whether not supporting unmatched surrogates is a bug or a feature—for a text editor I’d lean towards “bug”. But the two should be joined together, anyway.
vim/vim#2800 may be of assistance in resolving this.