5Dmk2 Live view Exposure simulation stops working

Probably an old topic but cant find any info.
I have 2 x 5Dmk2's and when i use Live view and with 'Exposure simulation' on (for camranger / ipad remote viewing combo), its ok for a bit then Exposure simulation stops working, no matter what i adjust the live view only show an average light levels.
First i assumed it was a camranger related problem, tried a different camera, same thing happened. So tried it just on camera live view, and happened on both. Starts off ok, then ES doesn't simulate.
Also i thought it might be because of also using a wireless TX in hotshoe for speedlights, so tested without anything in hotshoe, still eventually happened. Im stuck.
Im now assuming its a camera bug?
BTW im always using canon L lenses
Does anyone else have that problem, or have any idea what causes it or how to fox it?

thanks people

Sort of need this working as using in conjunction with camranger and iPad.
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