Did you submit a ticket? That's how they handle this stuff.
What happened, was you got paragon part of the way into the month? If so, then what you’re experiencing Is not a bug. You didn’t have the last objective when you started completing it. Meaning it didn’t register, obviously. Now that you’re paragon, you have the objective. And you’ve already completed it. Meaning you can’t technically 100% a quest that’s already 100%. Hope that helps you understand how these things happen. Just like carinas. If you don’t have the objective. You can’t complete it. That being said, the ticket should be able to get it solved. Submit one by clicking the setting cog-wheel