I'm just going to copy and paste most of a conversation that I had with another member, because it's pretty late.
If someone's brought this to light already, I apologize. The only thing I found via search function is someone complaining that this same issue (not having a date in the US release field) will refuse to show a review - which just adds further strength to the below.
A game is never searchable/linkable (or at least I can never do it) unless there is something in the US Release Date field. I have begun throwing whatever date is available in that field, even if it never released in the states, just because it'll pretty much be just as well have not been added at all, as it'll only be visible on things that have been linked from that page. Either it's a bug or it's some bizarre recordkeeping method that makes little or no sense.
Personally, I believe it should just be a release date reflecting the first release worldwide instead of the US, because things can get out of whack easily. Say Giant Bomb Adventures 1 came out after Giant Bomb Adventures 2 in the US for some reason (like it was never released here initially, but later only on Virtual Console or something), or maybe GBA1 came out in the US but not GBA2. It would look reeeeallly messed up on the timelines of consoles or series to have part 1 come after part 2. This gets very confusing. Also, when you don't put a date in at all, the game gets shuttled to the top of a list and therefore you've got a massive heap of random games populating the top of the list, mostly reserved for newer titles, with stuff that came out in Japan in like 1988.