The blueprints are saved separately from your game progress. So yeah, you may have left the game without saving. I think it's safest to go back to the menu before quitting to the dashboard or turning the system off. Again, it's been so long that I don't remember the quirks of this particular title. Sorry you lost some progress.
Usually it saves every few minutes or when something happens. But I usually do just turn the Xbox off when done right in the game. Maybe I did it when it was trying to save and corrupted it and it reverted back to a previous save? Don't know.
I had another bug earlier in the game. I unlocked the laser in the Jiggoseum, but it didn't show up on my trolley. I Googled that and I actually found the answer in a post you made on another forum (complete with the same sig pic you use here)... you have to beat the prior Grundy challenges to truly unlock the later parts.
I'm surprised Rare made such a big mistake there. It seems like it's a pretty big problem because I saw a lot of people asking about it.
One thing I found odd about the game was the box vs. tray. You finally get the box well after you could really use something like that. I had tons of different types of vehicles with a large tray with cages built around them to hold more objects, large objects, and to keep them from falling out. When I finally got the deeper box late in the game, I exclaimed: "I could have used this ages ago!" But I still had about 50 jiggies to get (not to unlock the last level, but to get all of them), so I thought there was still some use. I ended up getting about 30 more jiggies before beating the game, but I did most of it without needing a vehicle with storage capability. I also wish the trolley could get upgraded to a box instead of the small tray. Annoying, to say the least.