After iOS 15.5 update my iPhone's battery health dropped fast

I don't think such bugs have been reported. From my experience, however, I can tell you that with the heat your device drain faster and charging a hot device can affect the health of the battery. To preserve the battery and prevent the deterioration, during the summer you should remove covers and accessories during charging. Also avoid, whenever possible, using your phone while charging and keep it above 20% and below 80%. Also in the settings you can activate optimized charging.