This made me think of an article I read in DAN’s magazine a year ago. Long story short - this guy was on a dive boat, they saw dolphins and everyone quickly geared up and jumped off the boat. In his excitement, the man failed to fully turn on his air. He ended up running out of air but with plenty of pressure in the tank like you did...and he had to do a CESA from about 50ft.
The article went on to explain that this can happen if you fail to open your tank valve all the way. You can go to DAN’s website or give them a call to find the original article or get more information about what mechanism in the tank valve allows this to happen. If anyone has any insight on the latter - feel free to chime in.