Player score does not default to 0 in objectives.
Example: /testfor @p[score_id=0] can only find a player whose score has been set to 0 either by command (/scoreboard players set

id 0) or by score increment.
The command block output says: "The entity UUID provided is in an invalid format"
A bit more detailed: When a player first spawns in, his/her score would automatically be set to 0 for every objective. But now, each player needs their score to be manually set to 0 by command or score increment. This breaks many command block contraptions as the target selector score_

=0 does not work with fresh players.