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?