Yeah, my first thought was the "three letter words don't count" problem (not unique to XenForo by the way - it is common across the board, no pun intended). It has something to do with the computing power that would be needed for 3-letter strings, I guess. (Don't quote me, I am ignorant about such things.)
Anyway, if you can't make it work for you, feel free to post here about what sort of thing you are looking for. Maybe someone with good search-foo can help.
So basically I tried to search past posts about ex-wni (or eks WNI) technically it's more than 3 letters..but nothing came up. Maybe the search string/parameter (don't know the exact term) needed the words to be exact?
I tried searching on three keywords: "former Indonesian citizenship" and many threads came up - the first ones were not about ex-WNI, they were about how to gain Indonesian citizenship. But there are several pages - you might try that and go back further to see what comes up.