Unfortunately battery reporting is up to the device and not the HE hub. It is very inconsistent and super frustrating. My Iris motions die around 15-20% but my Centralite/Samsung water sensors will report 0% for over 4 months. This topic has been discussed and there are community solutions to help you know when they finally fall off the network.
Here is a recent post with community solutions mentioned:
I personally use an external solution using NodeRed to send me a Pushover notification every 12 hours letting me know if a device hasn't reported in the last 12-24 hours. More complicated than it needs to be for most but it is leveraging a database I already have in place for external logging.
