I went and got a bunch of Ripe Purians because I didn’t know how many you actually needed. (Got 6).
Then I proceeded to go find two of the silver lock boxes that were on the map. After placing the fruit and opening the Silver Strongboxes like the quest states. It still says: 0 / 1 Silver Strongbox Opened
I have opened all of the boxes on the map just in case, but regardless, there are no more Silver Strongboxes left for me to open in Bastion for the quest.
Either there is some instruction that is missing from the quest text on the actual objective, or something is bugged out with the actual quest progress tracker. I have gone back to Elios to see if it was actually completed, and it was still a grey ‘?’.
UPDATE:
Just so everyone knows. It is NOT ‘any’ Strongbox, it is the one that is IN Hero’s Rest by the stairs to the left of the quest giver. It is not the one that is directly below the required box on the lower cliff edge.
I thought it was the one that I did find before, but it was this one tucked in beside the stairs in the corner that you have to open to progress the quest…