I don't have any knowledge of how the Alembic electronics are actually set up, but here's an idea that makes sense to me.
A simple way to blend the two signals into mono is with a couple resistors - this causes the single output to be the average (sum divided by 2) of the two signals. In stereo mode, the two outputs bypass the resistors (they aren't in the circuit anymore). This has the advantage that there isn't a huge jump in volume when both pickups are switched on in mono mode.
So it sounds like normal behavior to me.
As an aside - if you get a chance to record this bass, record in stereo and pan the two channels slightly apart, listen in headphones to what happens. The sound will become subtly "three-dimensional". I don't have a stereo Alembic (yet) but I've done this with my Rickenbacker, it's magic!