I’ve noticed the same. I’ve tried it with Filmic Pro where I can set the shutter and iso, looked pixelated in low light. The iPhone app did the same. However there are pros shooting movies in low light, why do theirs look good? High iso?
Anyway, an update: I just recorded some video with filmic pro app, it looks fine. 4K at 30 FPS. No pixelation seen. But I did see some pixelation in really low light, lower that what should be used at 4K/30 FPS. I think the iPhone camera struggles in rally low light when shooting video because of high iso. Apps like Filmic Pro help with that by the user controlling the use and shutter manually.