iPhone 12 mini Dolby Vision HDR Playback dark...

Hi everyone, I may well be doing something completely wrong but I need some advice. I recently switched from Android back to iPhone after absolutely falling in love with the form factor of the iPhone 12 mini.

Today I went out for a walk with my little girl and thought I'd record some videos of her playing. They looked great so I thought I'd see what this Dolby Vision HDR was all about. Now I recorded the video and was excited to watch it back. When I did, the playback was pretty much black, Literally could not make anything out. It looked like a video shot in complete darkness.

I have attached a photograph of my phone screen with a video I quickly recorded to show exactly what I mean.

Please help as I am now dreading the thought of purchasing a faulty device.

Any help would be great and hugely appreciated.