Problem with featured image

When all images on post are from external url, e.g. addef with “Insert from url” option – post thumbnail missing on home page.

See this site for example, post have images
https://znaeteli.com/%D1%85%D1%83%D0%BC%D0%BE%D1%80/20-%D0%B3%D1%80%D0%B5%D1%88%D0%BA%D0%B8-%D0%BD%D0%B0-%D0%B4%D0%B8%D0%B7%D0%B0%D0%B9%D0%BD%D0%B5%D1%80%D0%B8-%D0%B8-%D0%B0%D1%80%D1%85%D0%B8%D1%82%D0%B5%D0%BA%D1%82%D0%B8/

but on home page missing thumnail
https://znaeteli.com/