I had a similiar experience today - after getting a "grand prize entry" piece, one of the boxes filled in on the dining board. Later I got another dining piece and completed it.
After that, all my pieces were "grand prize" entry".
I think what is happening today is two fold:
First, they have a glitch with the system again.
Second, it seems to me that why we are getting so many reports of "grand prize entry" symbols all of a sudden is that the program is designed (I think) to give you this piece once you complete a category (if you actually get a piece for that category). In other words, once you complete Dining, if another Dining piece comes up, it does not show that, but rather you get the grand prize piece. As folks fill up their categories they therefore will get more and more grand prize "substitution pieces.
Sounds like a good theory anyway.