It would be great if the scoreboard API docs page could mention the range of valid characters that need to be supported.
Somebody on Fore! Track leaderboards has an underscore character in their name, which was a surprise.
The playdate account says when you try to enter invalid characters:
"Enter a valid username. This value may contain only English letters, numbers, and @/./+/-/_ characters.”
Thanks to @stuffbyrae for help with this