After summoning a mob with a custom name, if you try to use Scoreboard functions on them, they display the UUID instead of the custom name.

To replicate:

Enter the following commands in order:
/scoreboard teams add Test
/summon Zombie ~ ~ ~

/scoreboard teams join Test @e[type=Zombie,c=1]
Instead of saying
"Added 1 player(s) to team Test:
Something"
it says the UUID.