Steps to Reproduce:

1. Summon a Wither;
2. Go through a Nether portal;
3. Come back to the dimension you spawned the Wither in;
4. Hit it until it's at half health.

Observed Results:
Once it reaches half health, the bossbar shows it as if it had none.

Expected Results:
The bossbar displays the Wither's health accurately

Notes: Changing dimensions again fixes the issue.