Continuity of the chain requires a credible transmission link between each narrator and the one above. This remains a primary requirement of hasan li-dhatihi. A missing transmitter cannot be repaired merely by the general trustworthiness of the remaining narrators.
Suppose A reports from B, B from C, and C from a Companion. If evidence proves B never encountered C, there is a discontinuity. Searching for another independent route is a separate task; the isolated chain should not simply be declared intrinsically hasan without proof.
Likewise, an ʿan-formula must still be examined under the applicable principles of meeting, hearing, and possible concealment of an intermediary.