Hi JKL037,
We appreciate you reaching out to us.
We’d like to gather additional details that may help us be in a better position to assist you.
- Have you had the chance to restart your iPhone? Restart your iPhone - Apple Support
- Are you experiencing the issue on multiple videos?
If you haven't, let’s check for a software update and make sure that you are running the latest version of iOS 16.
While it isn't inherently assured that an update will resolve the issue, updating to the latest software version for your device can help isolate potential software issues.
Kindly secure a backup and update.
- How to back up your iPhone, iPad, and iPod touch
- Update your iPhone, iPad, or iPod touch
Then, use the steps below to change the slow-motion section of a video shot in Slo-mo mode
- "Open a video shot in Slo-mo mode, then tap Edit.
- Drag the white vertical bars beneath the frame viewer to set where the video is played in slow motion."
You can use the article below as a reference.
Trim video length and adjust slow motion on iPhone - Apple Support
Take care!