وَإِنَّمَا كَانَ تَفَقُّدُ مَنْ تَفَقَّدَ مِنْهُمْ سَمَاعَ رُوَاةِ الْحَدِيثِ مِمَّنْ رَوَى عَنْهُمْ إِذَا كَانَ الرَّاوِي مِمَّنْ عُرِفَ بِالتَّدْلِيسِ فِي الْحَدِيثِ وَشُهِرَ بِهِ، فَحِينَئِذٍ يَبْحَثُونَ عَنْ سَمَاعِهِ فِي رِوَايَتِهِ وَيَتَفَقَّدُونَ ذَلِكَ مِنْهُ كَيْ تَنْزَاحَ عَنْهُمْ عِلَّةُ التَّدْلِيسِ.
Translation: The early critics were particularly concerned to verify hearing when a narrator was known for tadlīs, so that the risk of a concealed intermediary could be eliminated.
A known mudallis creates a specific risk of concealing the actual source of a particular report. That individualized risk differs from treating every merely possible indirect route as a proven or disqualifying omission.