in the surface on some of the mouldings. I can repair with a slurry of
cement and marble dust which sands out quite well and maintains the
shaping very crisply, but the delightful sheen that direct contact with
shutter confers is lost and the surface becomes somewhat dull and
lifeless.

Can anyone advise me how to avoid this occasional surface cracking on
the profile?

No, but I'll bet that if you don't stop at sanding, and go to
polishing the patches with 600-1000 grit wet sanding, you can
get back to a shiny surface again.