The bug

Unlike all other commands that target resource locations, the autocomplete suggestions for /advancement will not suggest vanilla (minecraft) advancements if you begin typing the start of the ID. Submitting the command will still correctly identify the advancement. This also applies to dimensions (possibly related to MC-135392).

How to reproduce

1. Type /advancement grant @s only
2. Press space and observe the complete vanilla list
3. Type adv (the start of adventure/adventuring_time)
   → No suggestions are shown
4. Clear the chat and type /recipe give @s
5. Press space and observe the complete vanilla list
6. Type aca (the start of acacia_boat)
   → All vanilla recipes that begin with aca are shown as suggestions