The change happened in the new version of OpenAL-soft. Please refer to this page for further information, and potential workarounds, as well as to this comment from [Mojang] Georgii Gavrichev.
The sounds are too much related on the player's view, what causes a mono sound if a sound is applied perfectly at right or left.
Stereo Sound Offset Is Too Strong
Snapshot Game Sound : Very low quality!
Audio panning too aggressive
Incorrect sound positions
Audio Issue - Stereo panning is too intense depending on where the player is facing
Minecraft Sound is too directional (still)
New 1.13 Sound System
Stereo moves between left and right speakers too quickly after 1.13
Jukebox/Note Block volume is too low when not facing it.
Sound is Directional
When jukebox is playing, turning you character will change sound level.
openal-soft #129: Center Channel Problems with OpenAL-Soft above 1.16
openal-soft #211: Sound has strange filtering depending on direction head is facing (Minecraft)