The Fit:Cover setting works in every view except the Mobile Portrait view:
https://preview.webflow.com/preview/chriss-fresh-project-1f6b30?utm_medium=preview_link&utm_source=designer&utm_content=chriss-fresh-project-1f6b30&preview=e9c346733b0b85394330dbf182dee6f9&workflow=preview
try making your lightbox height 100%
hope this helps