Most of these at least in my region are made from cornstarch. They decompose well/without "microplastics" but only under correct conditions.

Home composts aren't usually meeting these, their temperature isn't going high enough for full decomposition and you can have traces of polymers left behind. I throw them in the trash for compostable waste because thankfully my collectivity collects these to generate biogas and my guess is they do end up in much larger/managed composts where they can fully decompose.