I have completed the Druid Legion Class Mount long time ago.
I can also go to the barber and select it as my flight form.

As you can see in the screenshot i am in that form.
Also it says: "Already known" - but not collected.

The last quest in the chain (https://www.wowhead.com/quest=46319/you-cant-take-the-sky-from-me) that gives the mount also return "true", when running: /run print(C_QuestLog.IsQuestFlaggedCompleted(46319))

This happened to me as well as it was marked completed in all versions as of a month or two ago, but not since then. Wish I could remember which version.