If a player dies while the Health Boost effect is still active, the hearts from the effect will remain upon respawning, even though the effect itself is gone. The hearts will remain like this until the world is exited. This bug appears to have originated from snapshot 19w35a.
Thank you for your report!
We're actually already tracking this issue in MC-160405, so I resolved and linked this ticket as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
If you haven't already, you might like to make use of the search feature in the future to see if the issue has already been reported.
Quick Links:
📓 Issue Guidelines – 💬 Community Support – 📧 Customer Support – 📖 Game Wiki