This is great discussion, thanks.
When I charge deep cycle batteries at work for emergency backup power at telecom sites, the initial charge to completion will take it over its normal resting state initially, then it will "settle down" to its expected resting, fully charged state. For instance when first taken off a complete recharge it may register as 13.8 vdc, and over the next hour it will slowly drop down to about 12.8 vdc where it will remain while on a shelf for weeks or a couple of months with little change.
I have no idea if this is relevant to what you're seeing but I think so. I'm certain it's nothing to be alarmed about, unless a fully charged battery dropped its range dramatically to half of what is expected everytime its taken off the charge. Obviously I'm shooting in the dark here, never having even seen a FFE in person.