Well, I can second that, just happened here too.
- "Add new shared files with auto priority" is enabled
- File was in download, upload priority was auto (I think)
- I changed upload priority to very high while the file was still downloading
- After completion the upload priority of the file was changed to high
edit:
I just checked the rest of my shared files that got completed recently and it looks like (although i'm not too sure about 3 of the 12 files we are talking about here) that the upload priority gets set to what the files download priority was...