Question in 5x Crop mode , 5D2

[Ragnaros]

Hi,everyone
We use 50hz lights in China
So I set shutter speed 1/50s ,everything is ok

But when I enter 5x crop mode, there are shadows moving like this (not so obvious)








I find shutter speed has changed in crop mode
so I set FPS 23.976-low light   to make shutter speed keep 1/50s in crop mode
It seems good.But there are still shadows moving fast if look carefully, especially make some effects in Lightroom or Final Cut

Then I find  there are shadows in all FPS and shutter speed in 5x crop mode,except 1/33s 25fps-excat    (in fact 1/30s)
but 1/30s is too smooth to watch



How to solve this problem? or where is my mistake?
Thank you~~


I am from China and I'm not good at English ,maybe I can't describe exactly，sorry

My camera is 5d2， ML version 2016.12.2