The Bug

Note: L can also be any keybind that is set for your instance of Minecraft for opening the Advancements window. As L is the default key used, I am using it here for simplicity.

When pressing the L key on the keyboard to open the Advancements window while in-game, the game pauses as expected and the advancements window shows, but the music continues playing. However, pressing the Esc key pauses the game and the music, and clicking on Advancements in the pause menu shows the advancements window while the game and music are paused.

The same issue occurs when flying with an elytra. When pressing the L key while flying with an elytra, the flying noise continuously plays while the advancements window is open, even though the game is paused, but it does not happen when pressing Esc and then Advancements.

The video shown is recorded on 1.20.5 Release Candidate 2 launched from the official Minecraft Launcher. The same issue occurs on 1.20.4 as well. The video only shows the music issue, not the elytra flying issue, but it works in the same way; follow the instructions below to reproduce.

How to Reproduce

1. Open/make a new world in singleplayer.
2. Wait for some music to play or run something like 
   /playsound minecraft:music.creative music @s ~ ~ ~ 999999. For elytra, fly with an elytra and wait for the flying noise to play.
3. Press the L key to open the Advancements window and note that while the game does pause, the music keeps playing. For elytra, while flying, press the L key and note that the game pauses, but the elytra flying noise continuously plays on loop.
4. Press the Esc key and then the Advancements button, and note that the game pauses and the music/elytra sounds do not play.

Expected Behavior

Music/elytra flying sounds should pause when opening the Advancement window in-game using the set keybind to match the behavior of when pressing Esc and then Advancements.