The reason why Apple's Bluetooth headset charging red light is flashing:
Maybe the battery of Apple Bluetooth headset is broken, the battery is too low, the flashing light, the battery is charged, it does not flash full of light out. It is normal for Bluetooth headphones to flash when charging.
The charging case battery is low and may flash, not enough to fully charge the headset. Plug the charger into the charging box and charge it.