When a non-player entity are added to objectives, its UUID is displayed as its name in display slot.
However, when the objective executes auto display name update and the entity doesn't have any custom display name, the name of its entity type are used as display name.
Step to reproduce:
Create new world
Run following commands
scoreboard objectives add test dummy "test" scoreboard objectives setdisplay sidebar test execute summon minecraft:armor_stand run scoreboard players add @s test 1 scoreboard objectives modify test displayautoupdate true execute summon minecraft:armor_stand run scoreboard players add @s test 1
The first armor stand will shown with its UUID and the second armor stand will shown with entity type name.
What auto display name does is synchronize the display name with the entity name every time the score changes. Player scores are stored by player name while non-player's are stored by UUID. By setting it to true, you're telling the objective to show the mob name instead of the actual score holder name.
This is not a bug.