My MVC and Vistana reservations seem to frequently disappear from my Bonvoy account "Trips" listing. That happened recently with all of my 2026 stays so I went to add them back yesterday.
I found that I could not do so from the web site; it would show me sort of a broken version of each reservation with a bunch of missing info, and would not add that trip to the list. But I WAS able to add them using the iOS app on my phone or iPad, and once added, I could see the reservations on the web site as well.
So currently, all my trips are again showing up everywhere they should be. Until they disappear again.
As an aside: today I booked an Encore stay for November, tacked on to the end of an ownership stay. Interestingly, the reservation email says "MVW INT EXCHANGE" under the Summary of Charges. Usually Encore stays say something like "WVC MARKETING PREVIEW." I wonder if this means they are using other owned inventory (like developer-owned Abound inventory) for Encore packages. Doesn't really matter to me, I just thought it was interesting.