social.og-image-altThe share image has no alt text
Why it matters
og:image:alt describes the share image to people using a screen reader, and is what shows if the image fails to load. Without it the most prominent part of your link preview is silent. The Open Graph protocol says: "If the page specifies an og:image it should specify og:image:alt."
How to fix it
Add next to your og:image tag, describing what the image shows (not "share image" or the file name):
<meta property="og:image:alt" content="A three-tier wedding cake with fresh flowers" />