The effect is caused by wind shear.
KSP doesn't have wind! I hear you cry!
If a completely symmetrical craft isn't pointing exactly prograde then drag caused by the wind hitting the side closest to prograde is the result.
The higher the airspeed the greater the shear, the greater the effect.
The fins on the first craft produced more drag on the prograde side than the retrograde side which helped to 'steer' it toward prograde.
Very close to prograde the difference may be small. It depends on how much surface area is exposed to the shear effect.
Stray too far from prograde and any craft will stall and flip.
This is the reason some craft tumble when going into orbit.
The upside is that if you make a craft close to the shape of a bullet without fins, it will be quite happy to keep flying toward prograde and will allow you to let it stray further from prograde than the same craft with fins would do at the same speed.
If anyone else experiences the same or indeed any problem I suggest that you verify the installation.
Just because it doesn't crash when you load it up doesn't mean it's right in the head.
After verifying the installation it seems to be working as advertised.
I will continue my vanilla career with the updated version of KSP.