Gizmos render with an offset, but editor still displays the "right" transform.position

I use OnDrawGizmos() to render some lines and the object position. It works perfectly when in edit mode:

You can see that object Sector1 is selected and that position is (-1671.7, 0, 1746.5) in the inspector and in the scene.

Then I run the game and it still works OK, until I instantiate a prefab (a building) that has nothing to do with the Sector object (I 100% verified that it does not change the transform.position of the Sector object).

As soon as there is a new prefab in the scene, the Gizmos draw in the wrong location, but editor still displays the "correct" location:

You can see that object Sector1 is selected and that position is (-1671.7, 0, 1746.5) in the inspector, but the gizmos are offset and the position displayed by gizmos is (-2300.7, 0.1, 2869.2).

I was searching for a bug (I thought that somehow the transform.position of the Sector object changes in the code) until I noticed that the editor still displays the "correct" value for the object.

Does anyone know a possible solution for one of these questions:

(A) how to detect the EXACT line of code (call stack) where transform.position changes?

or

(B) how could instantiating a new object cause the change in transform.position of another object?