I understand that an anamorphic lens "stretches" an image when it is recorded, but when it's played back, it's "un-stretched" so that it looks normal. But with images shot with anamorphic lenses, everything in focus (i.e. subjects) is un-stretched, but out of focus areas, especially bokeh are still stretched (elliptical). Why?
Out of focus highlights effectively get squashed twice by the lens, once by the action of the anamorphic element (that is reversed when the image is re-stretched) and once on account of the round lens barrel that appears to be an oval since the camera "sees" it through the anamorphic element.
There are many tutorials on the internet that show you how to alter the shape of the bokeh by putting a mask on the front element of the lens:
By putting an anamorphic element in a round tube on the front of a lens you're doing the same thing. If you manufactured an oval tube for the anamorphic element you could get normal round bokeh.