OpenTk Transformations Example

Question

I'm running the Transformations example of the OpenTK and placed an alpha image instead the original one, and made some rotation from 0 to 90 degrees and back.

I can see an annoying trail when I move fast (about 60fps):

I've tried: VSync on, off, Renderfrequency, Update frequency, etc..no solution.

My GPU is nVidia RTX 3070, updated drivers, CPU is I9-10900kf. OpenGL version is 3.3.0.

Any idea?

the code is too simple:

if (!toDecrease)
{
    rotationAngle += 5.0f;// (float)Math.PI;
    if (rotationAngle == 90.0f)
        toDecrease = true;
}
else
{
    rotationAngle -= 5.0f;// (float)Math.PI;
    if (rotationAngle == 0.0f)
        toDecrease = false;

}
transform = transform * Matrix4.CreateRotationZ(MathHelper.DegreesToRadians(rotationAngle));