iPhone 4k Video Recording Actually Recording HD. Why?

I set my iPhone 7 up for 4k, 30fps, high efficiency video recording mode in the settings panel. It is also visible in the camera section that it is set to 4K. The thing is, while this is the setting, sometimes it records videos in 4k, 30fps, and sometimes it records them in 1920x1080, 29.98fps. I can’t seem to figure out when it will do which. I’m also confused why the frame rate also changes. The files themselves are an mov wrapper, h264. Does it have something to do with the length of the recording? It seems to generally change the resolution for longer videos, but sometimes it does not.

I have plenty of space on my iPhone, enough to record 4k, h264, and my photos are not connected to the Cloud so this should be the one and only copy of these videos. How can I avoid this problem in the future? How can I make sure that when the video is set 4K it records in 4K? And if it is somehow the system shrinking the resolution to save space on my phone, how can I change that? I would prefer it to just say, no more data and stop recording as opposed to changing the entire make up of my video.