The scores of custom players are not shown in a tellraw command. For example, if I run this command:
/tellraw @a ["",{"score":{"name":"nonexistent_player","objective":"test"}}]
I get the message "No entity was found", even though a score is set for this "fake player". I also get this message if the specified player isn't online.
I am not sure if this is the intended functionality, or if it is a bug. I would wish for this to work though.
I also didn't test it in other versions.