Here are two images with the 800mm PF. PF lenses can have an odd bokeh from specular highlights, but it's a very limited case that I have only seen with photos of backlit shorebirds using the 500 PF. I have not tested the 800mm PF in that scenario.Can you post some images demonstrating the "weird bokeh against foliage" please?
My experience is that bokeh against foliage often looks very weird even with normal optics, such as my EF 100-400mm. The reason for this particular weird bokeh usually has nothing to do with the optical construction, it's caused instead by heat haze, which can cause major deterioration in image quality when photographing mid-distance subjects on warm days.
Here is a 100% crop from the second image