Unique Strongbox did not drop anything?

Came across a strongbox, killed everything and nothing dropped? It isn't spawning anymore either and I looked around the area not too far though and no monsters are around either. Not sure how to add a photo but it was a
"Ixchel's Torment Armourer's Strongbox"
Guarded by 30 packs of Monsters
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Guarded by Aberrations of Chaos
Contains an Additional Unique Item, Corrupted with an Enchantment
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Corrupted
'His trials will end when time does...
And then, they will begin again.

Again I didn't die during the encounter nor have I portaled out because nothing has dropped and I was really hoping for that unique.
Thank you in advance.

UPDATE:
Apologies, while making the post I had the game open and when I went back I checked the corners and side rooms again and the monster that was left was now there so I supposed I just needed to wait. Not sure why it wasn't there before but it fixed itself. Thank you!