iPhone Xs max slow motion issue produces garbled footage, is this normal?

I know that slow motion should always be filmed in good lighting. But when I use the stock camera app on my iPhone Xs Max to shoot slow motion in low light, the footage is completely pixelated and unusable. In daylight it's ok, but still looks like its struggling with a limited bitrate (the bit rate seems extremely low and like it can't keep up). Not complaining about the noise which is understandable given the high shutter and low light. But it's the bit rate that seems to be so limited. These very poor results only occur in slow motion mode. I posted a clip here on Vimeo:

iPhone Xs Max slow motion issue

But I have no such issues when using Filmic Pro in slow motion. Is this just a limitation of Apple's stock video app on iPhone and not the iphone's camera itself?