I vaguely recall from the ArticFox log that they fixed a display issue on some multi-cell mods at some point (there might even be some parameter to offset that) hence why I asked if it was only on display. But if the cell tests low externally then it's probably something else.
Are you sure it's not the cell itself? I imagine you don't have the necessary tools but measuring it's internal resistance and comparing it to the others in the batch could shed some light. A weak or damaged cell would definitely produce the behavior you witness.
Could you try replacing that particular cell with an equivalent one, at least temporarily? Also how are the cells arranged in the mod? In series? In such configuration the mod shouldn't be able to draw on any single cell more than the others (as it's not discharging through the balance taps, only through the single +/- terminals) So it could be more the cell than the mod that is the culprit.
I don't have much in the way of testing equipment but I did try swapping the battery positions around and it seems to be bay 3 that drains faster no matter which cell is in it. All four batteries are LG HG2s that were purchased together a week ago and only used in this mod.