When breeding horses, the offspring's health is off from what is displayed when ridden. This would occur with either too much hearts or too little. When too much health is displayed, some of the hearts are permanently black in order to show the horses actual max health. When there is too little, the horse would have invisible hearts. I have a horse that has 15 hearts, but only 11 of them show. This occurs with a lot of the horses that were bred.

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