You want the result document ordered like you mentioned conditions in the query. For response time optimisations databases are allowed to list records in any order unless instructed otherwise. So you need to give the right sortby statement.
But you might test something else (I don’t know if this works):
Use several queries and have Tamino glue the responses together. This time the statement should look like this:
_xql=/News[matainfo/@ownerid=5] | /News[matainfo/@ownerid=12] | /News[matainfo/@ownerid=1]