No map data on special zoom levels

[zipferlacke]

The terrain-background map sometimes has no data, whereas the terrain map still has data.

Concretely the problem occurs here (http://maps.stamen.com/terrain-background/#11/51.4566/10.1102)
Here the map is white
While here: (http://maps.stamen.com/terrain/#11/51.4566/10.1102) data is still displayed.

This phenomenon can be seen in the whole area around these coordinates and is present in all maps. Particularly with the roads, where partly with certain sections also the data are missing over all maps which are offered. Depending on the zoom level above or below, the data are then but there, but in a different resolution.