15 Pro Max switching from normal to slow motion without me asking it to

When shooting a 5 minute, 30 FPS interview, the video speed switched by itself to slow motion after about 10 seconds of recording and then switched itself back near the end. The iPhone 15 Pro Max was on a tripod and no one was touching it. I was shooting in cinematic mode, inside in a highly controlled environment. Focus and exposure were locked. It happened in two of the six clips recorded. Why did this happen and how can I stop it from happening again? (I am very experienced shooting iPhone videos and never had this happen before. However, I rarely shoot in cinematic mode.) I see a similar uncommanded speed shifting complaints registered a couple years ago in the community, but no answers.