Moin,
indeed this is an (in my experience so far the only) error or at least misinformation in P'08 manual.
Outputs A and B do not separate layers, but voices!
If there are plugs in output B, voices 1-4 always and in any mode (normal/stack/split) route to output A, voices 5-8 always and in any mode route to output B.
As stated above this can be verified by LEDs: Upper row = voices 1-4 -> output A, lower row = voices 5-8 -> output B.
And yes, this results in lacking of voices 1-4, if output A stays free and only output B is used.
This is true for all modes and equals separating layers for split or stack modes only.
In "normal" mode this splitting persists, resulting exactly in Your (myert) observation. And it is always true, no matter if voices are triggered by keyboard, sequencer, external MIDI source or whatever.
This oviously is routed in hardware (or firmware at least): You cannot change this by any P'08 programming available to a user.
Martin