Why didn't Imam Shafi'i name Imam Ahmad when he narrated from him, and sufficed with saying "the trustworthy one narrated to me"?

The phrase "A reliable person narrated to me," or similar expressions, appears in the words of hadith scholars for reasons such as forgetting the narrator's name, doubting them, their being unsatisfactory, or for tadlis (deception). Imam Al-Shafi'i frequently obscured his shaykhs in his Musnad with various phrases, such as: "Some of our companions informed us," and "A reliable person informed us." It appears that this was due to his caution in narrating hadith. Al-Bayhaqi explained this by stating that Al-Shafi'i used to say: "Do not narrate from a living person, for a living person is not safe from forgetting." Or perhaps he doubted the one who narrated to him but did not doubt his trustworthiness, and was content with theeta's widespread reputation among scholars, especially since Al-Shafi'i may have written his books without having all his original sources available.