Flying Blue (Air France, KLM, and Other Partners) - Can't Change Multi-City Booking In MMB?

I have a fully-flex ticket for a multi-city itinerary, on both KL and AF metal (booked using the multi-city on klm.com, although I could also have booked it on airfrance.com ).

I need to change one leg of this itinerary (the fare rules say it is possible; it's well in advance, and there is availability; and I haven't booked any extra options or made any changes that would prevent further online changes). Unfortunately, when I go to 'Manage My Booking':
- On klm.com, the 'Change your ticket' option is greyed out (for all legs)
- On airfrance.com, it says 'This reservation is not eligible for online modifications'

I have a separate ticket, for a very similar routing (but booked as a return on airfrance.com, not a multi-city itinerary on klm.com) and when I view that reservation, the 'Change Your ticket' option is highlighted and available.

I am inclined to interpret this as meaning that "multi-city itineraries cannot be modified online". (It can't be a POS issue, as both have the same POS, and I doubt it's a ticket stock issue, even though one is 074 and one is 057).

Is anyone else able to confirm this? (Apologies if I've missed an existing discussion, but I couldn't find a thread anywhere).

I suspect the vast majority of AF/KL's pax fly on straightforward return (or one-way) tickets so this is not a huge issue for them - but personally, I find multi-city functionality very helpful. So, it'd be unfortunate if you can't change multi-city itineraries online.

Has something been changed to this itinerary before? I often notice that when something out of the ordinary has been done (ie return date changed) the itinerary can then only be changed by hand over the phone or ticket office.

Has something been changed to this itinerary before?

No - no changes made since it was originally booked.