I have set up a raster layer using a TIFF file for surface water flooding. The layer indicates the minimum and maximum flood range of 0.102624m – 0.47962m under the Layer Properties Histogram, but I can also see spot levels outside of this range (e.g. 0.9114m) using the 'Identify Features' tool. The data supplier is a well recognised company and thinks this might be a QGIS issue.
The TIFF file came with accompanying tfw, tif.aux.xml, tif.ovr, tif.ovr.aux.xml, and tif.xml files.