Start with a SCUBA dive with tank pressures already entered:
Click edit, as if to change something about the bottom gas:
Notice that it populates the Starting and Ending Pressure fields.
If you leave either or both of them as-is and click 'Save', it will change them to '0'
:
The workaround is to always re-enter both values every time you click 'Save' on that screen, even if you don't change either of them.