Some questions, for example
Instead of using a correct URL, such as
What is causing this behavior?
The Stack Exchange Title-to-URL-slug converter cannot handle characters outside the BMP (the Basic Multilingual Plane), running from U+0000 through to U+FFFF.
The character in question, U+1F60B, is encoded to UTF-16 using a surrogate pair instead, so you get U+D83D U+DE0B, and those are not 'displayable' (not valid Unicode) and get replaced by two U+FFFD characters on conversion to UTF-8.
The correct UTF-8 conversion would be
F0 9F 98 8B, so the URL should be:
which my browser then happily displays as
(if an Emoji is displayed depends on your font and OS).