H2SO3 is dissolved SO2. If the pH is too low, the SO2 is driven out of solution and it's stinky. (I'm on a stinky roll these days.) If the pH is higher, then the SO2 stays in solution as the sulfite ion, SO3--. The SO3-- ion is a good oxygen scavenger, which is one of its main uses in developers, and it is converted to sulfate, SO4--.
In other words, metabisulfite quickly becomes sulfite in solution. If the pH is high enough, there's no free SO2 gas dissolved in the solution. If the solution is acidic enough, you get back the SO2 gas from the dissolved sulfite and you can still have some sulfite ion in solution, but there's not some sort of "metabisulfite" ion present at any time in the solution.