Views style field loads the same graphics for all rows

I'm setting the dynamic style in an overlay loading an image stored in a taxonomy field.
I have a field with the image and I set the field style to 'Openlayers Style'. In the overlay settings I then set that field as style field.

I can see the image in the map with the style specified in the field as expected, but when there is more than one point the displayed image is the same for every point. The image loaded is the one in the first row.

In the view preview I can see a new field 'style' like that:
'style' =>
array (
'pointRadius' => 6,
'fillColor' => '#EE9900',
'strokeColor' => '#EE9900',
'strokeWidth' => 1,
'fillOpacity' => 1,
'strokeOpacity' => 1,
'strokeLinecap' => 'round',
'strokeDashstyle' => 'solid',
'graphicWidth' => 32,
'graphicHeight' => 37,
'graphicOpacity' => 1,
'labelAlign' => 'cm',
'externalGraphic' => 'http://sentieri.dev/sites/default/files/intgeo_home-2.png',
),
'wkt' => 'POINT (8.8875 39.1439)',
)

The field with the image is correctly loaded in the preview, but the value for 'externalGraphic' is the same for all rows.

Thanks
Filippo