Platforms like Facebook and Instagram will often default your post to a square format. This means that if you're posting a landscape video, the sides will appear to be cut off.

When posting, you can tap the "<>" icon to toggle between square post and landscape post. Here's the official documentation from Facebook for this  https://www.facebook.com/help/instagram/1469029763400082

Alternatively you can also create a square video on Lumen5 as shown here:  Creating a square or vertical video.