Objective criteria health does not register for non-player entities

/scoreboard objectives add DummyHealth health
creates a scoreboard objective which should be able to track and display entity health. This works for players but not for entities, especially mobs like the Ender Dragon or Elder Guardians.

To test, use these commands, along with a nearby mob:
/scoreboard objectives add DummyHealth health
/scoreboard objectives setdisplay sidebar DummyHealth