Bred horses usually display having 9 or 10 hearts while not actually having that amount of health.
When they do display it correctly, usually at full health, damaging them while make them display 10 or 9 again while the rest remain invisible.
The first horse has a maximum of 15 hearts, while the second has 14.5 and respectively show these two examples.
We're tracking this issue at MCPE-165899, so this report is being resolved and linked as a duplicate.
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