I have been using the Color Split component in v5 and have found that pages (from InDesign) that contain
(a) Registration black defined as CMYK 100, 100, 100, 100 or
(b) Index Color images defined as CMYK 0, 0, 0 100 (originally from Photoshop) or
(c) PANTONE Black C spot colors
are included in the resulting Color Inserts PDF rather than the Mono one.
Is this the expected behaviour?
That's the behavior.
You should be able to use a preflight profile to remap Registration (spot color All) and PANTONE Black C to K. If you can use a preflight check to detect CMYK 0,0,0,100 index color images then you can use a preflight profile to convert those images to gray. That should get those pages remapped to the monochrome job.