You're missing that you need to be in M mode.
Tv is shutter priority which means you set the shutter speed and let the camera decide other settings (aperture and ISO) to get proper exposure. The HDR video is cycling between 2 different ISOs so it's messing up the cameras auto exposure calculations.
Example:
Camera in TVYou set- Shutter speed 1/50
Camera meters lights in scene
Camera sets- f/5.6 ISO 100
Load ML HDR videoSet ISO at 100 & 800
At 100 ISO f/5.6 is fine as this is what the camera read
But when the camera meters at 800 ISO it's going to stop down the aperture to darken the image
Thanks, for your advice, it works fine in M, just trying to understand why it can't keep up.. One of them supposed to be under or over exposed. What am I missing?
You answered your own question here. It's not supposed to keep up, but in Tv mode you're saying that it should try to make a correct exposure