Hello!
I created my own server using BisectHosting and for some oddity, all users on my world have unlimited oxygen. When I checked the lung capacity command using /worldconfig lungCapacity, it was defaulted to "4000000". I have since changed this to 20, and used "/player Slik entity maxoxygen 20" to restore my oxygen to normal, but this hasn't changed anything.
I have also wiped my player data on the server and rejoined, but I still have unlimited oxygen.
Any ideas?? I'm happy to provide a mod list and or config if needed!