Kaomoji

Western emoticons such as :-) are read sideways and use a handful of punctuation marks. Kaomoji are read upright and draw on a far wider set of characters — Japanese kana, phonetic letters, box drawing and mathematical signs — which is why they can show arms, tears and whole gestures rather than just a face.

Because something in the destination treated one of its characters as syntax. Markdown reads backslashes, underscores and asterisks as formatting and removes them, and autocorrect can replace straight quotes or collapse repeated spaces. Pasting as plain text, and checking both edges of the face afterwards, avoids almost all of it.

A kaomoji usually mixes characters from several Unicode blocks, and few fonts cover all of them. The device falls back to a different font for the parts it cannot draw, so widths and baselines stop matching and the face stretches or sits crookedly. The characters are unchanged; only the rendering differs.