Tilemap coordinates issue - GetTile and SetTile access different tiles

I am using a simple script that reads mouse position and converts it into coordinates for the selected tilemap.

 public class MouseHandler : MonoBehaviour
 {
     public Tilemap tmap;
     public Vector3Int previousMousePos = new Vector3Int();
     public Vector3Int MouseTilePos;
 
 
 
     void Update() {
         Vector3Int mousePos = GetMousePosition();
         if (!mousePos.Equals(previousMousePos)) {
             previousMousePos = mousePos;
             MouseTilePos = new Vector3Int(previousMousePos.x, previousMousePos.y, 0);
         }
     }
 
     Vector3Int GetMousePosition ()
     {
         Vector3 mouseWorldPos = Camera.main.ScreenToWorldPoint(Input.mousePosition);
         return tmap.WorldToCell(mouseWorldPos);
     }
 
 }

Later on another script uses the value for the GetTile() and SetTile() operations with the same tmap. Problem is, SetTile() sets the correct Tile when given MouseTilePos, but GetTile returns the tile which is positioned several tiles above the cursor position (both x and y coordinates are off). I haven't got any idea how that can occur...

I am using isometric tilemap, cell layout is isometric with z as y.

I can't find the solution in reference documentation and must be doing a simple mistake that I can't see...