Battery warning/Hibernation on exhaustion of D-Dock Battery?

I've got both the internal (6-cell, 11.1V, 4700mAH, 53WH) battery and
the D-Bay (11.1V, 4320mAH, 48WH) battery in my new Latitude D600
laptop. The laptop uses the D-Bay battery first, then when that's
exhausted it uses the internal battery.

I have the laptop set up to warn on 10% battery and hibernate at 3%.

Last night I was running a slide show, and the computer went into
hibernation when the total battery power was at about 50%. When I
powered it back up again, it had a warning about critical battery
level, and that I should switch to outlet power immediately. The
D-Bay battery was at zero percent, while the internal battery was at
100%

It looks like it went into hibernation when the first (D-Bay) battery
was exhausted, without bothering to notice that the internal battery
was fully charged.

Is this a known problem, have I got something set up wrong, or what?

Many thanks in advance for any advice!