In function setVisibleRegion(), the input parameter shape is tested against internal m_region, if they are the same the internal fitViewportToGeoShape() function is not evaluated because the map is apparently already at that location:

if (shape == m_region)
    return;

Internal m_region is however not updated when the center or zoom level of the map changes (using setCenter() and setZoomlevel()). So a second call to setVisibleRegion() with the same shape as input always results in not evaluating fitViewportToGeoShape() even if the map center or zoom level is changed.