Yes! That’s correct!
According to your image that is the behaviour you should expect.
The mirror modifier works by default based on the origin point of the object.
When you added the circle, you moved it to the wheel location, this should be done in object mode.
When making location transforms in object mode, the object origin moves with the object.
When in edit mode, you are only move the vertices contained within the object, not the object itself. therefore the origin does not move.
Hope this makes sense.
