The background in Motion is transparent. When you export as an image, the png will have a transparent background and displayed in Preview or other viewer that has white backgrounds, it will look solid white. The Jpg version will just have a solid white background. If your "subject" is white... it just blends with the rest of it.
If you need a different color (or black) then you have to provide a solid color background (generators > Color Solid) OR —
you can select the project layer and then in the Inspector, the Properties tab. Look down til you find Background Color and below that is Background. Change it from Transparent to Solid or Environment and then export. I prefer to use Environment (it's supposed to interact with "the scene" (in 3D: lighting, shadows etc... in 2D, blend modes, etc.) [Example:]
This is a Current Frame JPEG export with the Project background settings set to Environment and the background color set to dark cyan. Changing the project background is easier than adding a color solid generator and is not dependent on project size.