Because he died before he was consecrated, he is considered only a pope-elect rather than a legitimate pope.
In 745, Stephen was made a cardinal-priest by Pope Zachary.
He died of a stroke only days later, before being consecrated as bishop of Rome.
[1][2] According to the canon law of the time, a pope's pontificate started upon his consecration.
[3] Later canon law considered that a man became pope the moment he accepted his election, and Pope-elect Stephen was then anachronistically called Pope Stephen II.