Welcome to the WDB!
The Fantasmic Dining Package has changed recently, so some info you'll find online might be a little outdated. What has changed is that now Guests are asked to go through the regular queue line for Fantasmic rather than being allowed to go backstage and circumvent the queue. Also, you need to be aware that you are not buying a specific seat (or even a guaranteed seat) for the show; rather, you will be seated in a roped-off section that is designated for those who had the FDP for that night. However...if you arrive too close to showtime, you might find that your seat has been filled by someone who didn't have the FDP. Cast members begin filling in the empty seats as showtime approaches, and if you aren't there you could miss out on the seating you thought you were reserving.
The biggest reason this is occuring lately is that Fantasmic shows were cut back significantly due to the budget constraints seen recently. Fewer shows each week means larger crowds on the nights when the show is performed. Larger crowds mean you need to arrive sooner...which takes away one nice feature of the FDP you mentioned. Instead of waiting until 30 minutes before showtime, now I recommend to my clients to show up 45-60 minutes ahead to guarantee that they will be seated. It's a conundrum whether the FDP is worth it now--on one hand, the shows are more crowded and seats are at a premium; on the other hand, if you're showing up early anyway, you might not need the FDP to get your seat.
Hope this helps!