The Jews don't believe in the Council of Jamnia. That makes it an imaginary council, with imaginary motivations and results. IOWs, it's fiction.
Any discussion of the Jews as single, monolithic block that all agree on anything is specious. In the first century there were at least four major competing Qahals each with a different canon; the Pharisee, the Sadducee, the Essene and the Hellenistic.
Peace be with you
Well, there are some historians who are skeptical... but precisely because there’s no mention of a Hebrew canon until after c. 135! What we do know is that at the time of Jesus, there were loads of versions of the Old Testament. And the extant ones (Septuagint, Dead Sea Scrolls) include the deuterocanonicals. Back to you.