iPhone frame rate too high

I took a bunch of videos last night on the iPhone 12 pro max. For the first 20 or so, the video plays back normally for the first few seconds then suddenly becomes slo-mo. When I look at the info for the video on the iPhone it says something around 117 FPS. The problem resolved later. The videos with the bad (high) frame rate have a circular symbol in the upper RH corner of the info window on the iPhone rather than the usual video symbol (a camera). Can someone tell me what happened and if it can be fixed?