I do something similar although rather than push the query results directly as you are doing, I start out with a blank multivalue token, then underneath the query result I do a for each row and push the value into the token.  I then add the token to the form exactly how you do with the query results and it works fine.  I'd note that I didn't do it this way to overcome your issue, but because it was an existing workflow and I wanted to keep the values I was sending separate from those that previously existed.  Not sure if I would have encountered your situation had I pushed them directly as you did.