The tranny does have an Overload Pressure Switch that will cut the timing above 1.8 bar.
This can happen under full load downshift [ your complaint]
The sw is OPEN at pressures less than < 1.8, so Randy may want to disconnect it for testing only.. [

On the 4K rev limit in N , that is torque converter protection ..
This is supposed to be speed sensed over-ridden when you move > than 5 mph..
A test for this circuit is to get the car moving [ I use 20mph]
and manually shift to N . Then test the RPMS ... the 4K feature should be eliminated under these conditions...
Randy prob knows this anyway, but ...
See what that gets you