Found a Reversion Since the RiiConnect version

[intra0]

registering a new mii with special characters in their name doesn't work (specifically tested with the << ⭐ and >> characters)
It gives a 257304 (invalid nickname), this happens when both registering a mii artisan and uploading a mii publically

Interestingly if you registered during the riiconnect days, this was not a problem and you could upload miis with the's characters freely.
Even more interestingly is that if you still have a save file and the associated unmodified mii from those days, even on the newest version of the check mii out channel, both your mii artisan and uploaded mii will still be valid and even show their special characters online

I have not tested if the same issue affects Everyone Votes but its a possibility