If the Username you set the player_mob to is more than 16 characters, the game crashes to the main menu.

To replicate you can do:

/summon player_mobs:player_mob ~ ~ ~ {Username:"12345678901234567"}

This results in the "Connection Lost" screen: https://i.imgur.com/zCjzZaj.png

Normally this would not ever happen, however some folks twitch user names are longer than 16 characters. My CCI setup uses their twitch user name unless the twitch user had changed that via a channel point redemption. I have had to put in a quick work around on my side to chop the username at 16 characters.