Some scholars have been cited as accepting a trustworthy transmitter's addition without making a distinction. Yet unconditional acceptance even against a stronger contradictory transmission would undermine the requirement that a sahih or hasan report be free of shudhudh (disqualifying anomaly).
The statement “a reliable narrator's addition is accepted” cannot therefore be applied as an absolute rule that disregards conflicts, differences in precision, and the evidence identifying the best-preserved version. Distinguishing compatible from incompatible additions resolves that problem.
The narrator's reliability matters greatly, but a demonstrated contradiction to stronger evidence can still affect the ruling on the specific addition.