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....
Can you please give an example of what names did not function as expected? Keep in mind that only letters (a-z), numbers (0-9), and underscores (_) are permitted in the username bar.