Facebook is always changing things. The Link dialog is for sharing links and if that link has an open graph card in the page header, that page wants you to use the image they have provided. For instance here is part of the header (the <HEAD> tag) from a local newspaper article:
\<meta property="og:site\_name" content="newsobserver"/\> \<meta property="og:type" content="article"/\> \<meta property="og:title" content="Hurricane Florence forces high school football teams to adjust to short schedule"/\> \<meta property="og:url" content="https://www.newsobserver.com/sports/article218330220.html"/\> \<meta property="og:description" content="Several area schools, including Leesville Road and Garner, had a quick turnaround after playing on Friday because of Hurricane Florence"/\> \<meta property="og:image" content="https://www.newsobserver.com/latest-news/2xb0qb/picture218330955/alternates/LANDSCAPE\_1140/RAL\_%20091218\_GAR\_LEV\_FB\_SW\_1\_2%20(2)"/\>
So if you share a link to Facebook and there is an og:image tag in a page, Facebook should use what’s there. I am speculating, but perhaps they were using your provided image if there isn’t an og:image tag or instead of and then changed it so that if there is an og:image tag they will use that first instead of your provided image. Again, I’m just guessing, but it’s a logical guess.
If you want to share an image of your own, perhaps you should use the photo dialog instead of the link dialog.
Rob