The lithium banks are primary (non-rechargeable) batteries.
I'm thinking it's temperature related. The lithium bank is dead, but it's voltage will vary based on ambient temperature. Colder temperatures will lower cell voltages, and warmer temperatures will raise them.
Either that or it's due to bank loading - over the winter, the systems would have been trying to draw from the lithium bank, but as mentioned its dead, so the voltage would collapse rather than support any load. Now that the lead-acid is taking a charge, they're completely unloaded.