Did you get to solve this problem?
I'm having a similar issue.
My download only works once and don't know why.
You cannot download and redirect at the same time, this is simply a limitation of the HTTP protocol (it has nothing to do with Seam or JSF): when processing a request either you provide a single response (a webpage or a download) or you redirect to another resource. You cannot do both at the same time.
Possible workarounds for this issue are: