I can imagine the problem resides in a refusal of the plane to drop
its nose properly and build airspeed when one wing is more deeply
stalled than the other. I'd guess a tendency to be too flat in the
spin and adding more rudder is not the solution. One thing that stops
yaw increasing is body section and the center of aerodynamic
resistance as the spin progresses. It may be that the skinny aft
section and a lack of boxiness to increase drag for yaw (and a low
inertial nose) makes things worse in that regard. As yaw rate
increases the spin starts to flatten...

My 2c.


I'll add another 1cent. worth.

Notice that most new designs that are spin resistant have their vertical tail
members well in front of the horizontals. That configuration places the fin
into clean air as the airplane is spinning and descending..

Where the fin is you could double it, and it would not do much better. It needs
to be out in clean air, not blanketed by the horizontal, IMHO.
--
Jim in NC