MAX_ACCELERATION value?

using current 2.8.
it looks to me that the actual axis acceleration is limited to exactly half of the value specified in [AXIS_X]MAX_ACCELERATION. attached is a picture with waveforms where [AXIS_X]MAX_ACCELERATION = 50000 (mm/s/s), which results in joint.0.acc-cmd swing between 25000 and -25000. is this intended behavior? not clear from the documentation...
also i noticed that corresponding [JOINT_0]MAX_VELOCITY and [JOINT_0]MAX_ACCELERATION have no effect, at least with trivkins.

I could reproduce in a sim for some planned (gcode or mdi) moves.
For instance, a single g0 move with enough travel distance to reach
full speed shows this behavior.

I think it is built-in trajectory planning behavior for some moves as
set here:

github.com/LinuxCNC/linuxcnc/blob/master/src/emc/tp/tp.c#L328

hm. it seems that it is not the full story... something pretty weird is happening on the last segment of the trajectory. code: