This is a planned behavior and happens because the map defaults to a fixed size in the scene.
When you zoom in/out, the displayed distance between map boundaries changes, but the map size in scene is fixed, so the height of the map changes.
Here is an example of how to fix the center point at the same height:
using UnityEngine;
namespace InfinityCode.OnlineMapsSupport
{
public class KeepCenterAtZero : MonoBehaviour
{
private OnlineMaps map;
private void Start()
{
map = OnlineMaps.instance;
map.OnMapUpdated += OnMapUpdated;
}
private void OnMapUpdated()
{
double tlx, tly, brx, bry;
double lng, lat;
map.GetCorners(out tlx, out tly, out brx, out bry);
map.GetPosition(out lng, out lat);
Vector3 worldPosition = OnlineMapsTileSetControl.instance.GetWorldPositionWithElevation(lng, lat, tlx, tly, brx, bry);
map.transform.Translate(new Vector3(0, -worldPosition.y, 0));
}
}
}