4-27 Using Composure output. Camera zoom don't sync with preview windows.

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I’m using BMD SDI composure output.
When I change the camera focal lenght.In unreal preview windows looks fine,but the SDI output to my TV monitor doesn’t sync.
but when I release my left mouse botton，Image in TV monitor change focal length immediately。

This is the video:https://www.youtube.com/watch?v=a_FoHopT5N4