I looked at the files above, in the world's stats and advancements folder, and found that I did have them all in the stats file (can be read with notepad) but was missing the elder guardian in the other. It gives a date and time each one was check marked and noticed there was nothing older than 2017. I think i had this world longer or maybe you need internet for it to update since i was playing offline a lot. But after looking at what was missing, I just killed another elder guardian and it updated. Hope that helps.
Michael Faulkner
I looked at the files above, in the world's stats and advancements folder, and found that I did have them all in the stats file (can be read with notepad) but was missing the elder guardian in the other. It gives a date and time each one was check marked and noticed there was nothing older than 2017. I think i had this world longer or maybe you need internet for it to update since i was playing offline a lot. But after looking at what was missing, I just killed another elder guardian and it updated. Hope that helps.