In the Figma documentation a guideline for custom file thumbnails is provided. The specs for custom file thumbnails show a safe zone. As per definition the contents of the safe zone should always be visible when the thumbnail is displayed.
When a user sets a Thumbnail based on the specs provided, contents of the “safe zone” are cut off, depending on the browsers and/or figma apps viewport size.
For a consistent presentation of the custom file thumbnail the aspect ratio of the thumbnails should be locked. At least, the implementation of the thumbnails should respect the defined safe zone.