Photoshop: Split channels on large CMYK files gives glitches when converted to bitmap or saved as tiff

This has happened to me on CS6 and now with CC on OSX running El Capitan. When I split channels on a large CMYK file the resulting files look fine, but when I convert them to bitmap or save them as a grayscale .tiff or .psd I wind up with glitched files. This does not happen if I cut and paste the channel into a new grayscale file, so I think it is a bug in the split channels code. Original CMYK file was 7393x5700 pixels

BITMAPPED WITH A HALFTONE SCREEN




SAVED AND REOPENED AS A GRAYSCALE TIFF


ORIGINAL IMAGE AFTER SPLITTING CHANNELS ( BLACK CHANNEL )