EDIT because the web form cut off my question after the character that causes the problem in yEd.
When I add a unicode "key character" (U+1F511) to a document, it is displayed correctly, but disappears when saving the document.
When I add 🔑 to a saved document using a plain text editor, and open the modified document with yEd, the key character is displayed correctly, but again disappears when saving.