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 )
I wasn't able to reproduce this creating my own CMYK file, splitting the channels and saving a tiff of each file using the default settings in the Tiff options dialog,
It only happens if the CMYK file is not flattened and split channels creates a transparency channel. I can create a new file the same size and there is no problem. Double click the background layer to make it Layer0, and the glitch manifests. I flattened the original file and now there is no glitch.