There is no reason to believe everything Paul wrote was inspired to the level of scripture. One of his sermons was recorded in Acts. Does that mean scripture is “missing” the other 2000 sermons of his that were not recorded in scripture?
Even the letters of Paul himself had to be discerned by the Magisterium, first. Some things are “obvious” only in hindsight.
There were other books of Acts, and countless other Epistles, that might have been included - chosen - in the Canon, but were not.
Paul is extremely important in the books canonized by the Magisterium under the guidance of God. Paul’s importance is not dependent on the selection of books for the Canon, but that is how WE know his importance.