This isn't a problem with chdir, it appears to be a problem specific to directory named "tmp". If I try to chdir to a directory that doesn't exist, then the "Browse" button shows me files in the working directory of the script. If I successfully chdir, then it shows me files in that directory. But on both of those cases, if the directory name is "tmp", then I am instead shown files in the "My Documents" folder. Sounds like just the kind of thing Microsoft would do.
An aside: If you cancel out of the Browse window, you get a two-button dialog box that either throws you back into the Browse window or exits the program. You really should allow the user to cancel the Browse window and return to the original form in case they change their mind about browsing.
(but my friends call me "Tye")