There is a glitch where if you wear a certain helmet in the water it can sometimes add a half bubble of air in the bar, since adding a image wont work for some reason I added the images in these links:
Air glitch:
https://ibb.co/mXzp0x
Helmet + enchants:
https://ibb.co/iZjNLx
This is a duplicate of/relates to another ticket in which an extra bubble will appear under certain conditions. I will try to look for the other ticket.