Friend tried to join my game and it said "Not a Valid Name"

My friend wanted to join my world and when he tapped it it loaded and then said "Not a valid name" Then when he changed his name to something else i.e "Banana_Pinappleluv" It let him join.

Recreation of bug:
1: Load up a World
2:Get your friend to try and join
3: Sometimes you will get "Not a valid name"
4: Change your names in settings
5: It let's you join

This is super weird....