When uploading a portrait image (taller than wide) to a landscape project (wider than tall), the system will apply a special portrait effect. This occurs only if the width of the image is 40% the width or less of the canvas. Generally speaking, a 9:16 ratio image will enable portrait mode, while a wider 3:4 ratio image will not.
The effect will use a blurred duplicate of the foreground image to handle the mismatch in dimensions. To avoid this, you can create a square video, or upload according to landscape dimensions.