Instagram links shared via iMessage have not been showing the link preview. An Instagram spokesperson confirmed this was a bug and it will be fixed (via Mashable).
Instagram Link Preview Bug
Like other iMessage link previews, Instagram previews let you see part of the Instagram post before you opened it. Instagram previews have been working fine on other messaging platforms, but something caused it not to work on iMessage.
Mashable wondered if this was done intentionally on Instagram’s part (owned by Facebook) and reached out to the Mysk research team. The team analyzed the network traffic between iMessage and Instagram and concludes it does look like a genuine bug.
When iMessage tries to open the link to look for the metadata, Instagram forwards the requests to the login page. Because the login page of Instagram has no metadata for link previews, iMessage doesn’t show anything.
Other messaging services don’t get re-routed in this manner, Mysk found. Perhaps the question of intentionality is still up in the air.