With the Health Boost effect (effect ID 21), if you log out with more than 20 health (10 hearts), the correct health value is saved, but upon logging back in, the health value is capped at 20 (10 hearts) and you must regain the health you had.
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Still a problem on 14w10c. At least when the hearts come from armor with Attribute Modifiers. Added a similar message on MC-17876
Duplicate of MC-17876, please use the search function to see if your bug has already been submitted. Currently over 57% of tickets are being closed as duplicate.