Biodegradable plastics are typically degrading in intense environments, such as a compost pile. Not unlike wood. Wood is strong and lasts a long time if kept dry. If you introduce the environment where fungi, microbes, and insects are more comfortable, the wood starts to deteriorate.

I don’t know quite how it’d work with this kind of plastic, but maybe someone will come up with an indicator or rule of thumb that helps. Like how you don’t eat food out of a dented tin can.