Hello,
Very interesting thread.
To add to Kev's post - OLD ribbon's watermarking (moiré) will often suffer if exposed to water (spit) etc. This can vary from partial to complete 'vanishing' of the marking.
Shall one want/need to clean their piece - try removing ribbon. This of course varies with each medal/order but in some cases jump rings are not sealed (soldered) and can be lightly bent to remove (together with ribbon). Of course, handling of old and often fragile ribbons can result in damage and we all know how hard it is to find old stock ribbon to replace (if even possible in many cases).
From my experience: typically, French and Swiss made ribbons are very delicate in that regard. German, Dutch and Belgian produced ribbons will 'take' it better. English, Austrian and Italian ribbons seem to be most durable.