Put the summary of the bug you're having here
What I expected to happen was...:
I didn't actually expect anything to happen, I was taking a cruise with my elytra.
What actually happened was...:
The game crashed, I'm not entirely sure why
Steps to Reproduce:
1. Make a new Minecraft World (Hardcore)
2. Crash the game (intentionally only to test, does not need to be intentional to trigger the bug)
3. All unlocked achievements/logged statistics (distance flown, # of mobs killed etc.) reset for some reason while everything else in the save was untouched.
Related issues
Comments

Thank you for your report!
We're tracking this issue in MC-187701, so this ticket is being resolved and linked as a duplicate.
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Cannot reproduce. I:
Opened 1.16.5 vanilla
Made a hardcore world with default settings
Mined a tree, made a wooden pickaxe
Dug down, got stone, made a stone pickaxe (two advancements gained)
Held F3+C to crash the game
Re-opened the game
Pressed L
My advancements were still there
What you experienced was probably fully anecdotal, and specific to your crash and its circumstances.
EDIT: also tested it cruising with elytra (that I obtained with commands). Gave myself all advancements, cruised and crashed it, my advancements were still there.