I think it’s the same issue reported here. My same comment:
Having the same problem with my watchlist (600+ items reduced to 10), which is then synced with my Plex Watchlist (via Plex Sync), which ruins my Plex viewing experience. I discovered that if I visit my watchlist on Trakt Web, all movies are there. But on the iOS App (and the API), it’s reduced to 10. This makes me hopeful that the list is still intact, but it’s a bug with their mobile/API layer. It’s been like this for several days now. Hope they know and are working to fix it.