Video Doorbell 2nd gen showing as hardwired with solar panel but not charging

Hi, I have had a solar panel charger for a week or two now and wanted to give it a while to confirm the behaviour before posting. When the panel has enough charge it is showing as hardwired which I understand as it's connected to the hardware terminals, but unlike other posts I've seen it is not charging the battery at all so the battery charge is gradually decreasing. It uses a micro USB port not C as the doorbell was bought a couple of years ago, but why is the battery not charging as it's obviously picking up the solar panel connection? Is the unit faulty as it charges fine via a USB cable?