It is important to understand that biodegradable plastic is not a single miracle material. This is a large family of different polymers. Some of them decompose faster, some slower, but they all need special conditions for this. The goal is for the plastic to decompose into mineral compounds as soon as possible and cease to be plastic at all. That is, petroleum polymers should not remain in the soil.
That’s just for this you need to maintain a certain temperature, humidity and access to oxygen. In the compost pit, this becomes possible (of course, if your package is hydro-biodegradable, not oxo-degradable), but not in the oceans and landfills. In Europe, where biodegradable packages are produced in large quantities, they enter special composting plants and are proud to take part there. But in Russia, it would be fairer to ban plastic bags from being biodegradable.