[quote]Acquiring the year 3 thorn does not unlock the grimoire card for the thorn.[/quote] You need the year one version for the card. Wait for Xūr to sell legacy primary exotics and go fishing. It's expensive strange coins wise, but it should get you the card.

I have the exact same problem. Completed the Abomination Heist strike, then unlocked the chest & received the yr 3 Thorn but the thorn grimoire card did not unlock & neither did exoctic weapon blueprint.