moire - Pattern shown in LED screen background used as backdrop on a stage

I'm trying to figure out an oddity in photographing a large LED background. I'm using a Canon 5D Mark II with a 100-400 lens although it occurs with any lens and at any focal length. The camera is picking up a pattern that isn't visible with the naked eye.

It looks like aliasing that you might get when photographing a LED monitor except here the screen is the size of a billboard. I'm not sure I would call it a moire pattern though.

The shutter speed must be 250 minimum, usually 320 or 400 due to action on stage requiring it otherwise action is blurry. Lighting is a bit low so ISO 3200 has to be used and aperture is wide open either 2.8 or 4.0 depending on the lens.

Obtaining a shallow depth of field can help by blurring the background a bit but it's still there. I haven't really noticed a difference in the angle I'm photographing as to if it makes a difference but a straight on shot is what is needed in most cases to capture the scene. A much slower shutter like 160 starts reducing the effect but it is too slow for action in the scene.

Using my Iphone 7 Plus, taking the same photo all lines are completely gone! The picture is 100% solid. That's using an Iphone 7 plus and an automatic photo (no settings). The only time the iphone 7 plus does not do well is if the action is in high motion, the action may be a bit blurry.

The only time I have been able to obtain a good photo with the Canon 5D Mark II (also tried a Sony A6000) and multiple lenses is when using a flash but a flash isn't really acceptable to fire off during a show. It appears with the flash firing it is indicating a shutter of 1/200 (which is usually too slow for action) but how the flash works I believe it is only firing for a fraction of the 1/200 even though I have it manually set for a faster shutter. The action is frozen and the screen is solid. Shooting at 1/200 without the flash yields the same results of lines in the screen. So I'm not exactly sure how the flash is causing the LED screen to appear solid but it works. The iphone 7 plus (no flash) works.

So how do I shoot in order to remove the lines in the photo of the LED screen?

Thanks... Gary