Nuhoch Tracking Lesson

Today I was working for the achievement "Lightbringer's Pack", I killed Potoni and got the Nuhoch Tracking Lesson. Then I deleted the item, since I didn't need it anymore (or so I thought), but it also disappeard from the Lightbringer's Pack progress. I killed him again, but I didn't get any drop (same day, so normal).Is it intended that I have to keep all the trophies until I complete the whole backpack?(I wrote here since support seems particularly busy in these months)

Do you mean an item you got for the Lightbringer's Pack (achievement)? I am not sure but I think the collection items go in the achievement list when you get an item and stay there. I don't see anything odd stated on the wiki page regarding at least. Apart from a "you can only do it with the character you started this collection with" and a "May not drop if the player requires more than one Nuhoch Lesson (dropping a different Lesson instead)."

That is odd, such items are immediately updated in your collection upon acquisition.Did you loot from the chest after battle, and do you have the Nuhoch Proving Mastery learnt?

Yes, I have all the 274 MP. I looted from the chest, and after that the Lightbringer's Pack achiev has been updated, I discarded the Nuhoch Tracking Lesson item. Then, when I got the following item, I noticed that the Nuhoch Tracking Lesson was missing again from the achievement :/I'll try again and this time I'll keep the item until I complete the whole backpack. I'm not sure if this is intended.