Some small important points which may help?
When we read the voltage of batteries it can often fool us if we are not aware. When reading the voltage, if we do it as the batteries are charging, we do not get the true state of the battery but instead only see the voltage which the charging is putting in! And to get closer to the true state of charge, we need to wait several hours and then check.
Maybe review what you are doing/seeing to see if you are actually getting as much charge from the solar as you think?
My thought is that you may be seeing the solar as doing all the charge needed but the battery itself is still gradually going down, then when the charge stops,the reading drops quite quickly.
Can I "mansplain" without seeming offensive? Sorry, not meant that way but batteries are confusing things!
They are a chemical reaction and might be compared to looking at a barrel of water. If we look in a hole at the top as we add black ink, it may look like the water is really black. That is something like checking voltage as we are charging. You only see what is right at the battery post!
But if we come back after a few hours of not adding ink (charging?) the chemicals will be stable and we may not see much black at all!
Techie term is "surface charge" and you may find much better explanations if doing a search.
I suspect there is a basic problem of the charge going in from solar is actually less than being taken out by use. So it appears good while you are putting charge in but over time the battery chemicals are really getting lower and lower until they no longer operate.