The first beacon advancement will not unlock if you place the beacon on a fully leveled block pyramid first. You need to put it on a smaller block level first to unlock it.
Comments 2
Unable to reproduce, a full pyramid can be used to unlock the first Beacon achievement with no issues in 20w21a/22a.
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While yes, it does look like the advancements are gone when you head to 20w17a, simply just holding a crafting table in your inventory will unlock all of your advancements again (as the crafting table is the way to get the Minecraft advancement, which makes the actual advancement menu appear.)