That's kind of the issue at heart. Most product management includes at least some aspects of project management.
The core problem I've seen is when the product manager sees the project management aspects of their role as their primary or only job. This is unfortunately incredibly common.
Well, the clue is in their names. Project managers manage projects, which could be anything. They juggle resources and timelines.
Product managers are responsible for the direction of a product. They have to figure out what customers want or need and ensure that the product being built satisfies some of those needs and is competitive in the market place.
There may be some project management in product management, but there is no product management in project management.
The core problem I've seen is when the product manager sees the project management aspects of their role as their primary or only job. This is unfortunately incredibly common.