I'm playing on bedrock using an android phone. The bar is going down normally however the damage I'm taking is happening twice as fast as it usually does. What I meant in the original post was the hp bar goes down twice as fast after running out of oxygen.
This just happened to me. I was building an animal farm and I spent about 90 minutes building it and suddenly the game crashes and when I logged back in everything was gone. I hope this wont happen again!
I'm playing on peaceful mode so there are no mobs I'm regenerating hearts and even then the damage I'm taking is faster than usual.