... The GoPro battery should not be getting charged. And on a properly functioning Solo and gimbal, it will not be getting charged. Ever. Not under any circumstances. It doesn't matter which GoPro you use or what firmware you have on the GoPro.
But it DOES matter which firmware version your SOLO is, because the very early versions (2015) did recharge the GoPro... or at least they SEEMED to, because they made its "recharging" LED light up, as we old-timers noticed and commented on here in this forum. And we noticed when that STOPPED happening (I forget which firmware update that was) and we commented on it here. Unless I'm hallucinating again.
If you are saying that the HARDWARE of the gimbal is simply not capable of recharging the GoPro, then why did its "recharging" LED light up in the early firmware versions of Solo? Maybe it wasn't actually recharging, but something else was turning that LED on? I'm open to any suggestion ... other than downgrading my Solo to an obsolete firmware version for the sake of science.