Hi, this thread is a few years old, but this is the exact problem that I am also having. Basically, the "export" process in gcloud sql DOES NOT export multiple tables (as described in the OP).
Does anybody else know the solution? Is it just a bug that has never been fixed? Hope someone can give me more information here as it is very frustrating.
Please do read the OP for more information. It is the exact same problem that I am having.