QGIS 2.6.0-Brighton - adding new fields to shapefile - screwed up borders

I am trying to add a custom field to my layer made from a shapefile of Italian provinces; everythins seems fine when I create a cathegorized views using regione codes, someting like this:

then when I try to add a custom filed to the associated table in order to create a custom aggregation (like the Macro Region shown on this picture:)

when I go back to the ma everithing seems to be screwed up, and this is the result I am getting

Am I doing something wrong or is it a bug??

thanks to all who may and want to help Anselm