Are user stories supposed to be problem definition of solution specifications

Are user stories supposed to be "Problem definition" OR "Solution Specifications"

In the organisation that I work in user stories are used for solution specifications. The acceptance criteria will describe how the UI should behave and what happens to the data once saved etc.

When I researched online, I came to understand that user stories should be problem definitions. I have some questions regarding this. 1. Even if user stories are problem definitions, I don't understand how acceptance criteria lies in the problem space and not in the solution space. 2. If user stories are problem definitions, what documentation should we use to define the specifications of the solution that the dev team needs to build.