Allay advancements are secret

Advancements being set to hidden is a feature of the game, vanilla only has a few of those. This seems intentional.

I agree with Dhranios that it is intentional, it is similar to how dimensional advancements work, they are only visible once you unlock that dimension and complete the advancement

In 1.19 Pre-1
I assume this may not be intended after all, as it was given an assignee?

Once a ticket has a "Mojang Priority" added to it (it's set to Normal here) it means that Mojang considers it a bug.

In 1.19.2 and 1.19.3 Pre-2

In 23w13a

In 22w14a

In 23w16a

In 23w17a

Still in 23w18a