Cassandra
Etymology
From the Trojan princess Cassandra, whom Apollo gave the gift of true prophecy; when she spurned him, he cursed her so that no one would ever believe her — she foresaw Troy’s fall and was dismissed as mad. The name now marks anyone whose correct forecasts go unheeded, so the sting is in the ignoring, not the error. This distinguishes a Cassandra from a mere doom-monger.
noun · /kuh-SAN-druh/
Someone who issues accurate warnings that everyone ignores.
What it names
The precise word for a truth-teller whose problem is not being wrong but being disbelieved.
Example
The risk analyst was the office Cassandra, flagging the exposure months before the crash hit.
Related idea
A Cassandra is right and ignored; a harbinger is simply a sign that something is coming — one is a disbelieved person, the other an omen.