Image with a caption text are surrounded by the [caption ...] .... [/caption] shortcode in the post editor. If those images are set to be aligned, eg center aligned, they still don't appear centered in the AMP page.

It seems that the aligncenter class (or other alignment class) is missing from the <amp-img ... </amp-img> block.

On the contrary, images without a caption (no caption shortcode) appear centered in the AMP page, if that is what has been set in the editor.