I'm guessing that the player who wanted to use this name ran afoul of a length limit, which was apparently imposed by byte count rather than character count -- e.g. maximum name length was 31 bytes, and this just happened to fall in the middle of a two-byte utf8 character, causing the last byte to be uninterpretable as utf8. (Whoever is responsible for imposing the length limit should revisit the issue.)

I think your method (in your later reply) of using a space to replace each "\x{FFFD}" (the unicode replacement character, which is inserted whenever there is an "uninterpretable" byte sequence) is as good as any, though maybe the "ellipsis" character ("\x{2026}" or "\x{22ef}") would be more appropriate.

IMHO, anyone who goes to the trouble of creating a "name" that contains both Latin-based (left-to-right) and Arabic-based (right-to-left) characters in a single word token is most likely trying to make trouble, and should expect (presumably wants) to see things go wrong.