vim wildmode=longest,list doesn't work when files are on different directories

Question

vim file name autocompletion using tab doesn't work if files are on different directory levels.

Steps to reproduce:

1. set wildmode=longest,list in vimrc
2. vim usa1.txt test/usa2.txt
3. Type :b u in EX mode, then press TAB. It is expected to auto complete the word up to ":b usa", which is the common header of two file names. But it just doesn't work.

Any solutions? Thanks a lot!

PS: if the above two files are placed on the SAME directory, it works!!!