It's because there is almost no life past the 5k mark, the pressure is too immense. To be completely honest it's a bug we just found out about today as well. And it can happen at any depth, happened to me at 4k mark, to Dans17 at 3k, and to our friend who play-tested it at 800m. We are looking into it. When it comes to the oxygen minigame, there is a lot more wonkiness with it. It's the only minigame that has not been completely/ properly implemented, was the last one to be added to the game. If you look closely at the art there is a slot on the tank where the bar feeling should be displayed but it stays empty. Also, the minigame works too close to how the fuel minigame works, it's basically a reversed version of it. On paper, you were supposed to fill in the tank on the right to its fullest and only then you will receive oxygen back to your tank. Sort of like how it happens with the pressure, only lowering when you complete the mini-game.
Now, holly shit man! What a mad lad! You spent over 44 minutes of your life on our small indie game. We couldn't feel more happy, proud, and honored to have a player like you. Thank you so much, you my friend are getting a spot in our humble hall of legends and mad lads! What a power move :D