When you increase your max health with /attribute then die, the hearts will not appear full when regenerating health and stay grey (with the exception of the normal amount of hearts). However, these hearts still act as health without showing the red.
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[MOD] Greymagic27
Thank you for your report!
We're actually already tracking this issue in MC-179940, so I've 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 to see if the issue has already been mentioned.
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