Sounds not transferring to headphones if plugged in during game-session

What I expected to happen was...:
Sounds should be transferred from the speakers of the pc to the headphones, 

What actually happened was...:
whilst the headphones work on other apps like discord, minecraft doesnt seem to register it, making the minecraft sounds play on speaker while everything else is on the headphones. 

(ps. tried on two seperate computers, same problem)

Steps to Reproduce:
1. Start up Minecraft without headphones plugged in
2. Plug them in after having started a save or server
3. Experience duality.