I have a hero header activated for my post pages and it all works well when I have a featured image. When I don’t have a featured image two logos are shown next to each other and the fallback image does not show.
It looks like you’ve uploaded a logo in the header element on the post without the featured image, but on the other post, it doesn’t seem the case. I think there might be some cache issue going on.
Can you check if there’s a logo uploaded to the header element? If so, can you remove it?