Custom sounds from resource packs cannot be played relative at an entity with /playsound

• When you have a resource pack with a custom sound in it, you can play it with /playsound [trackname] @a

• Place down an entity, for example, an Armor Stand, and use:
  /execute @e[type=ArmorStand] ~ ~ ~ /playsound [trackname] @a

• When you do this, it will play the sound at the player instead of the Armor Stand. However, when using a sound that is in the game by default (for example random.explode) it will work and execute that sound at that entity.

To easily test this out and not having to make your own resource pack, here is the download link for the resourcepack I used when I discovered this bug.

The sound is called spacemountain.music, so the command would look like this (when executing it at an Armor Stand, can be any entity):
/execute @e[type=ArmorStand] ~ ~ ~ /playsound spacemountain.music @a