Sorry to come back so late:
Take the village "La Mota del Cuervo" (Cuenca) in Spain: At zoom 10 there is no name displayed. At zoom 11 I can already read all localities around Mota del Cuervo but the village name is not displayed. At all higher zooms no name of the village is displayed.
Take Sevilla (Granada)): At zoom 7 the name is displayed or not depending on the exact map section. Often much smaller villages are displayed. Up to zoom 10 the name is displayed. At zoom 11 the name disappears though the city is still fully contained in the map section. Much smaller locality names are displayed, though. Zooming out again the name of Sevilla disappears at zoom 9 reappears at zoom 8, disappears at zoom 7, and reappears at zoom 6.
Strange and disturbing. Of course it has to do with the scaling of the fonts etc.
Beat