I looked it up in different dictionaries but could not find anything. Thank you in advance.
Bonne question ! Personnally, I write "reflet" for the noun and "être reflété" (to be reflected) for the verb. This may be an anglicism, but I can't think of something else.
You can use the word "réflexe", but the word "dérivé" is much more used.
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.