The preview was always monochrome, so the Print in grayscale checkbox appeared to do nothing but flicker between the colour thumbnail and the (wrongly) greyed sheet raster.
Move GRAYSCALE_TAG to a free tag bit (1 << 59) so it no longer overlaps PRINT_TAG, and let the colour and grayscale preview requests carry different tags so set_print_grayscale actually re-renders the right one.
Test: print_preview_tag_distinguishes_grayscale_from_colour failed before the change and passes after.