Any resolution? I’m seeing same… Disconnecting my solar panel now. 
Similar issue, although I’m switching between cameras. One camera charged as it should and once swapped to the other camera it drained from 97% to 12% during the day
The camera is NOT designed to allow continuous charging. The warranty is voided by attempting to do so.
I connected a power bank. The camera has some built-in logic to stop continuous charging. Shortly after reaching 100%, the camera stopped charging and went into normal burn down.
As for solar chargers, most are designed in a manner that can not keep up with usage. Similar to power banks, most can not both recharge themselves while also outputting to charger a device. It is one or the other. Solar panels can slowly recharge a device. Keep in mind, home solar systems can not keep the battery fully charged if all devices are continuously powered. People off the grid can only operate some lights and limited devices all day. They map out what major devices and appliances they can run and for how long.