So I have that blue completed bar indicating there's at least one I haven't completed yet. I've checked a quest list online and have them all. The seven special arena quests rotate in and I've physically seen six of the seven.

The only one I haven't seen is the Pukei-Pukei. I have since captured two of them as that is the prerequisite to unlock it (that and a high enough hunter rank which I have).

So either it's glitched by not unlocking the quest, or I have done it and it's glitched by not having the orange completed bar.

Anyone else have any other suggestion or reason I might be missing?

Edit: It was indeed Zorah Magdaros. Once I completed it, it completed the rank six optional quests. Thanks /u/Bobbyj1016.