Step to reproduce:

     1.Use locate command to locate a Trial Chambers in the world.

     2.Change game mode to spectator mode.

     3.Fly down to the Trial Chambers.

Obversed Results:

     You can get "Minecraft: Trial Edition" advancement while you're in spectator mode.

Excepted Results:

     You can't get "Minecraft: Trial Edition" advancement while you're in spectator mode.