I'm working on a project that involves filming playing cards, and then cropping/joining different parts of the video. For a seamless result, the background of the video needs to always remain the exact same, so a side by side of different timestamps doesn't look edited. I've found when I film, my iPhone camera is constantly changing the colour/lighting of the backdrop depending on what's in frame. This is not a result of any actual shadows that are created, I can see the table appears the same in real life while I film. I've linked an example video of this happening just by moving my hand in and out of frame.
I've tried several "raw" and "manual" camera apps which lock focus/exposure/white balance, but they all have this issue. Is this something unique to the iPhone? Would I have this problem with another camera? Please advise!