The bug

The ability for note blocks to play the sounds of mobs if the mob's head is placed on top is an experimental feature which requires a data pack to be present in the world to experience. Despite this, note blocks which play mob sounds can still be obtained in non-experimental worlds.

How to reproduce

A debug stick is the easiest way to reproduce this, as it can set the note block's instrument accordingly. However, debug stick issues are not supported. Another method exists, however, which does not require it:

1. Create or enter a world WITHOUT the 1.20 data pack
2. Execute the command

   /give @s minecraft:note_block{BlockStateTag:{instrument:"creeper"}}

3. Place the note block
4. Right click on it

Expected results

The note block would not play creeper sounds when right clicked, as note blocks playing creeper sounds is an experimental feature which should not be available on non-experimental worlds.

Actual results

It does.