The bug

Although the tab-completion suggests names, when you start to type the part of that name, that is after underscore, it does not do that with dots or slashes for example in minecraft:adventure/bullseye advancement.

How to reproduce

1. Type or copy the following:

   /playsound mu
   

   →  Notice that in list is suggested minecraft:music.credits

2. Type or copy the following:

   /playsound cre
   

   →  There is suggested nothing, even though there should be minecraft:music.credits