No Expert on this but, I think the solar charge circuit (mppt) is independent to the firmware
ie. the firmware only reads the values.
I’ve update 2 different types of solar nodes to v1.16 and not noticed any problems.
You could try reflashing the device with older firmware, the same way as you update
using https://flasher.meshcore.io/ and clicking the “firmware” pulldown to select the version required and see if this has any effect.