All I can pass on is a previous experience we had. Our first cruise was with Carnival on the Tropicale in 1991, two years later we did the Mexican Riviera on the Jubilee, then the next year a second cruise on the Jubilee. Tropicale surprisingly is still in service being leased by current owners for charters and as we speak Jubilee is being turned into knives and forks in Bangladesh. Down the road we were booking another cruise on Carnival and in bringing up our cruise history and as with you, Tropicale was not listed and neither was our first cruise on the Jubilee, maybe not a big deal but losing the two cruises meant we were not able to get a free upgrade do to the position it put us in in the loyalty pecking order.
So, like you, off to the Carnival i go, like you I get, "ship happens" and the more I thought, more I didn't like it so called back, this time got a gal, that said she felt my pain, asked her why, someone just stab you in the back?. Anyway, she really was helpful, she explained that their way of tracking past cruises was going through a big overhaul and as old as my cruises were they were just not picked up by the system, OK what do I do? She said best thing is to come up with whatever I could to prove I was on those cruises. Best thing you can get your hands on is that here we go again "boarding picture" but God bless it it gives the date and usually the cruise, and lets face it they don't really want to loose you as a customer and what's a cruise point to them, they bought it, at least for me.
The trick is to come up with stuff, any stuff that can prove you were on those cruises then go back and be nice but be persistent.
Hope this helps