On the current version of MediaWiki, adding ANY supplementary character litterally (such as 🕹 for the Joystick Emoji), i.e. directly in the visible form (encoded in UTF-8), but the page is TRUNCATED on this character, discarding all what follows !
The same page can be previewed correctly, clicking just the "Save" causes the bug where text is lost !
Bug seen on Wikimedia Commons, and on OpenStreetmap wiki, using Mediawiki version 1.26.3 (fa11b59)
This is a security issue (well the affected pages can be restored from history but all edits past the supplementary character are invisible/lost)
Don't close this as "invalid" ! Something is not documented and visibly this does not affect the history comment, or the start of the page;
this is something occuring after some volume of text. And the database is ALREADY using UTF-8 everywhere !