Potato starch bags do compost. A sample has been on a Bagbuster compost heap for a month, and after a lot of poking around looking for it, there are just shreds. The "100% degradable" ones are still intact after years, apart from some holes where couch grass roots have pushed through it.
This is a triumph. These bags look and behave like plastic when they're in use, but after that they just disappear.