The Jamnia council is a useful case study in how a historical claim can propagate without evidence, and it is worth setting out the mechanism because the same mechanism operates elsewhere in this atlas. Three ingredients were involved.
The first is a real institution. There was an academy at Yavneh, it was important, and questions about the status of particular books were discussed there. A story attached to something real is much harder to dislodge than an invention from nothing.
The second is a familiar template. The scholars who framed the account knew church councils intimately, and a council that meets, deliberates and issues a canon was the natural shape for the question to take. A model borrowed from one tradition was applied to another that did not work that way, since rabbinic Judaism had no body empowered to issue binding decisions of this kind and did not settle questions by that procedure.
The third is repetition. Once the claim was in the handbooks it was cited from the handbooks rather than from sources, and each citation made the next one easier. By the middle of the twentieth century it was possible to write about Jamnia without ever having asked what text says it happened.
The corrective is unglamorous and always the same: go back and ask which document attests the claim, and read what it actually says. Applied to Jamnia, that question has no satisfactory answer.