Check your proftpd configs, on all of my proftp servers (solaris 2.6 7, 8 and 9) mtime gets updated on file upload. I have no idea why your setup would be different -- in fact I don't even see how it is possible, FTP's STOR command does not transfer timestamp info about the file it is storing so how would it even know what the mtime was on the file on the client.
-Waswas
Edited: Here is the the like to the FTP RFC note a search for mtime atime and ctime all come up nil =)