I have used 2.18 to print maps to file (tiff) using print commposer. The base layer imported into QGIS consists of NOAA raster tiles. (My problem arises whether or not I have added other layers.) The composed maps seen in the composer look fine. However, the tiff file(s) created have problems. In particular, the tiff file output shows "sub blocks" of the image in composer moved from their original locations (e.g., scrambled).
My sense is that the problem arises from my use (misuse?) of raster tiles.
Any ideas regarding the nature of the problem and solutions/fixes?