I looked it up in different dictionaries but could not find anything. Thank you in advance.
Wiktionary says it's continuateur, but I couldn't really confirm this is actually a technical term after a cursory search. The French Wiktioanry defines the linguistic sense of English reflex as "Issue, continuateur ou aboutissement d’un étymon ou d’un phonème étymologique." There might not be an established counterpart in French.