Myth
Madison was terrified of having an Article V convention based on his experience at Philadelphia.
Having witnessed the difficulties and dangers experienced by the first Convention which assembled under every propitious circumstance, I should tremble for the result of a Second
- Madison to George Lee Turberville, 1788
https://founders.archives.gov/documents/Madison/01-11-02-0243
Fact
Madison was afraid of having another Philadelphia convention. He's not even talking about the convention in Article V in this letter.
At the time Madison was writing this letter to Turberville, New York was pushing for a second general convention before the new Constitution had been ratified. This convention would be Philadelphia, part II - an anything goes convention.