Hello katerink,
Thank you for reaching out to Apple Support Communities.
You were reviewing a helpful article. Have you restarted your iPhone since you first noticed this:
"How to restart your iPhone 6, 7, 8, or SE (2nd or 3rd generation)
- Press and hold the side button until the power-off slider appears.

- Drag the slider, then wait 30 seconds for your device to turn off. If your device is frozen or unresponsive, force restart your device.
- To turn your device back on, press and hold the side button until you see the Apple logo."
More details can be found here: Restart your iPhone
If the issue persists, can you provide a few more details? Does this happen with all videos or one specific video? Is the video downloaded onto your iPhone? Also, where is the video originally from?
Cheers!