Hi, I had the same problem with the Dragonvine Strap for another of the pack collections, as you can see in the topic I started yesterday.
https://forum-en.gw2archive.eu/forum/support/bugs/Dragonvine-Vigil-Strap-not-unlocking
However, your backpack will not turn exotic without completing the rest of the collection items.
The strap not unlocking even though you’ve finished the final instance is a problem though and I urge you to also send a ticket to Customer Service to complain. Initially they did not believe me, told me their system showed that my character had consumed the strap – so they could not help me. Despite all my screenshots showing that it had not unlocked the item in the collection tab.
After I’d done the story again on the same character AND brought a whole new character through the entire story for the same backpack and got nothing either, they sent me a replacement strap through the mail in minutes. That sorted it for me.
So, no use in running the instance again, nor with another char… just send them a ticket.
