Why sometimes the preview part of Facebook post sharing a post from StackExchange site remains empty? (By preview part I mean the picture which usually contains site icon. IIRC if the question contains some images, Facebook allows you to choose one them, too.)
I have first noticed this when trying to share a post on Facebook. The preview was displayed empty. So I tried to see whether this happens to other people who share posts from SE sites. I will post some examples below.
I have tagged this support rather than bug. It is possible that I am doing something incorrectly when sharing posts. And it is also possible that Facebook is to blame for this and not StackExchange. After helpful explanation by Shadow Wizard in the comments I have added the bug tag.
Here are some examples (with screenshots) showing that the preview (site icon) is sometimes displayed and sometimes not. First two examples are from StackExchange FB page. The next two examples show posts from MathOverflow. (I first searched for such posts, since I have first noticed this problem on a post from this site.) The fifth one is an example from March this year. (Just to show that this does not happen only for old posts. It is the most recent post of this type on Stack Overflow FB page.)