Hi there,
Problem is all of your featured images have a different aspect ratio. Ideally you want them to be all the same.
If thats not possible the you can force them all to the same Height in the Customizer > Layout > Blog –> Featured Images >> Archives. Just set the height property for the shortest element. It may then need some CSS to fix the Featured Post image.
Its not an ideal solution as this also ‘breaks’ WP src-set – which means responsive image sizes will no longer load for different device sizes.