That’s ADR and is telling your device to change its settings. Assuming the device is handling it, it might actually improve your battery life. See What are these port 0 downlink messages?
But if the device keeps getting the same downlink over and over again, then something is wrong. It might not receive the downlink, it might not process it, or it might be using ABP (or OTAA, with last join before October 2019) for which TTN has a bug in some fallback.