Slumbering Cerberus entered the battlefield with the blue chains usually saved for stun counters. Triggering its morbid ability (thus untapping it) does not remove the blue chain visual from the battlefield.

Playing on PC

The blue chain indicator is typically removed from a creature when it becomes untapped.

Slumbering Cerberus keeps the effect on itself (only visually) the entire time it is on the field, making it confusing to play against.