Bug with being underwater

I was mining underwater, and there was a lip of air (it was a stone wall, with a water pool on the floor next to it. I started mining into the wall, and so water rushed in. You know when the water isnt level anymore, so there was a lip from where the water reached the top and the next block above it.) that I went up to breathe in, so I stayed there thinking that I was above water because there was no "air bar" ( the bar that shows how much air you have underwater) I continued mining in the "pocket of air" when my hearts started going down. Before I could figure out what was going on, I drowned. If I really was underwater, why wasn't there an air bar?

If it is a bug, maybe you could fix it so that if you do go into the pocket of air, the air bar will appear so you don't drown expectantly.