After updating from 1.15.2 to 1.16 (the full release) I found that all my advancements had been reset. it gave me advancements that I already had in 1.15.2.
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Have you at any point opened the world in 1.16 or any 1.16 development version, and then opened the world after that in 1.15.2 again?

I only updated my world to 1.16 and opened it in 1.16. I didn't open it in any of the snapshots or pre-releases and I didn't open it in 1.15.2 either.

Are you using any custom data or resource packs?

nope, just vanilla minecraft

Resolving to MC-187701.