A Raster "GeoTiff" using 8Bit is always interpreted as Unsigned. So a Signed map is misinterpreted, a map with a range -128 to 127 but is interpreted as from 0 to 255.
Do you have available test data that demonstrates this problem? If so, can you link to it, or attach it if small?
Without a sample we'll have to close the issue as "cannot reproduce".