How far should you plan ahead when working agile/Scrum?

So, I am working in larger project for around 2 years now, and we have not recently hit a point, where our planning is actually on the same level as the current development progress.

Basically, each week, we do Backlog Refinements, where we go through our User Stories, try to understand and refine them etc. The problem is, that we are always talking about Stories, which are either going to be implemented in the next sprint already or which even are in the current sprint. This refinement is usually happening on a really high detail, because our PO does not really have an understanding of the requirements himself (yet he is only one knowing them at all), and we as developers need to try to understand his limited understanding.

This more than often leads to really vague stories, which will then be discussed in further technical detail in different calls, when we actually start working on them. Often we also realize in these sessions first, that the requirements are either completely irrational or out of scope, which in turn blows our sprints.

(We actually had to scrap 2 Stories today, which we are currently working on, because we have been told wrong requirements)

Here I am wondering… Is this normal? Is it normal to discuss the requirements the moment you have to start implementation? Or is our planning just super behind on the timeline, and would need to get further ahead?

Any thoughts on this are welcome, because this has become really frustrating in the recent time…