If an image mosaic with indexed colour rasters in a projected coordinate system (Transverse Mercator, Albers Conic Equal Area) is reprojected to another CRS, vertical discontinuities will appear. Immediately to the right of a discontinuity, the image will be correct and as it is approached from the left, the distortion increases. The degree of distortion appears to be proportional to the distance from the upper left corner of the mosaic.
To replicate, create a mosaic with images in the upper left and lower right corners, in a transverse mercator projection. View a portion of the lower right image reprojected.
Attached are two GeoTIFFs in EPSG:27700 and a section of the lower right image in the mosaic projected to EPSG:4326 showing the problem.