Colossi Infection Cave can't be Completed? A Bug?

I'm 1 cave away for my first Lootrun and I just need to clear the Colossi Infection Cave. Despite having cleared the 2 chest and got the Blue Big One (Usually Indicates the Cave Progress are completed), the Content Book keep saying that I haven't cleared it. I also check the Cave entrance and see the progress bar at 0%.

Eventually I tried to clear it 2 more times in different times (total of 3 completion) but still can't have it completed on my Content Book. Do you guys have any idea what I might done wrong or should do? Or is it just a bug?

*I've also seen a thread from someone else 4 months ago speaking about this, but there's no one replying to them

 

The Colossi Infection Cave has one end chest, and it's a T4. The way to access it is to hug the right wall of the cave upon entering. My guess is that you accidentally went into a separate cave upon entering Colossi Infection. It's an easy mistake to make because there's a few holes in the left wall that drop you down into a separate cave. Make sure you hug the right wall and look for a T4 chest, and it should complete.

 

I think the separate cave is probably the issue here. If the Content Book is still at 0%, it sounds like the actual completion chest wasn't triggered. I'd try entering again and sticking to the right wall until you find the T4 end chest, rather than dropping through one of the holes on the left.