Incorrect boss health bar when bosses are killed via the /kill command

Discovered while testing MC-257185.

The bug

When a freshly-spawned boss is killed via use of the /kill command, its bossbar will not read as empty.

How to reproduce

1. Create a wither, for example via a wither spawn egg

2. /kill @e[type=minecraft:wither]

3. Look at the boss bar while the wither is dying

Expected results

As the wither is dead, the boss bar should be empty.

Actual results

It remains completely full and then disappears entirely.