Hello Mariusz
I looked at your code and at the moments of inertia in your example...
It is because in your specific case, the intermediate axis (due to the geometry and position of the centroid) the intermediate axis is the one parallel with the blade (the psi axis). And it does not give such a nice and smooth ride??

The angular speed plot on the left is from the paddle in my previous reply, the plot to the right is from your paddle
You can see that in my case, the unstable rotation is around the theta axis (yellow curve), whereas in your case, the unstable rotation is around the psi axis (green curve)
BTW, it is not recommended to use subscripted variables in Mathematica code. It sometimes gives problems.
Thanks for yr interest in this case!