Canon 5D and flash question

I have the "mark I" 5D and noticed the other day that if I had the camera set to Av exposure mode and the 580 flash set to manual (I know probably not recommended anyway), the camera would lock at 1/8000s regardless of the aperture set. In Tv mode, the minimum aperture of the lens is locked in regardless of the shutter speed set. Manual exposure set on camera and flash appear to work as expected.

I've read through the manuals and a very good web resource on the Canon flash system but can't find any reference to this. I do seem to remember reading something about the issue somewhere and I believe it was specific to the 5D. Can anyone shed some light?

Thanks.