Featured image from srcset

Is it safe to assume that FIFU cannot set the featured image if there is no SRC for the images inside a post?

For example, I have in one post:

<img sizes="auto" srcset="https://www.tion.ro/wp-content/uploads/2017/09/+otf/650x435/pro-musica-a-facut-spectacol-la-timisoara-10-650x435-7.jpg 650w" class="article-cover-img article-img desktop">

On the index pages (front, search, tags) the FIFU Default Featured Image is displayed, not the one in the SCRSET. Is this normal behavior or is something I’m doing wrong.