I’m at the cave where the Illuminated Footlocker is supposed to be and have gotten the light buff a few times, but the chest isn’t spawning. I’m assuming that it erroneously has a single-loot respawn timer instead of being globally available for all players. This has happened in the past with other treasures so it seems the most likely reason.
Same thing happened to me. I did the cycle 2 times before the chest appeared.