There is a distinction between “primary” microplastics like you’re describing, and “secondary” microplastics caused by the breakdown of larger items.

From Wikipedia:

Two classifications of microplastics are currently recognized. Primary microplastics include any plastic fragments or particles that are already 5.0 mm in size or less before entering the environment. These include microfibers from clothing, microbeads, and plastic pellets (also known as nurdles). Secondary microplastics arise from the degradation (breakdown) of larger plastic products through natural weathering processes after entering the environment.