Do a test -> Transfer one the video to a macOS or Windows OS computer.
See this Apple article: Transfer photos and videos from your iPhone, iPad, or iPod touch
Transfer photos and videos from your iPhone, iPad, or iPod touch to your Mac or PC - Apple Support
Check if the video plays properly.
This is to find out if it is the hardware of the iPhone, since the iPhone 6 (iOS 12.5.5) is an outdated iPhone (9 september 2014).
If that video plays correctly, then transfer all video to a computer.
If all video then plays properly, then it would be the hardware of the iPhone.
Visit an Apple Store or an Apple Authorized Reseller in your region for assistance to diagnose the problem.
And they could offer a possible solution.
You will first have to make an appointment due to the pandemic.
OR you may want to consider switching to a newer type of iPhone.