An easy, but not 100% accurate way of ensuring this, is simply to right-click the video in Photos on your Mac and select "Get Info". This will show the average number of frames per second over the whole video. This is usually not exactly 240, but extremely close.
Obviously as it is an average it can vary very slightly from one range to another within the ...
Today, I went back to the Apple Store.
After running some tests, they were able to determine a problem with the telephoto lens and Face ID. (Curiously, they saw someone with the same problem earlier that day.)
Because my phone was still within its warranty period, they replaced it free of charge.
Your mileage may vary, but it's worth a shot!
There's a built-in setting in iOS that lets you configure Camera app to remember setting for Live Photos.
On your iPhone, go to Settings app → Camera → Preserve Settings and enable Live Photos.
With Live Photos set to On (as shown), it will persist the last set state between app launches. With it turned off, Camera will reset to Live Photos turning On ...