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 create a new square video on Lumen5, or resize an existing video from the share page.