As a shortened form of
Stephanie,
Stefanie, Stephanya, Stefanya,
Stephania, Stephanya it's become accepted. But even then it doesn't make sense to me, since the V (sound or letter) doesn't turn up in these names as it does in
Steven or
Stephen.
Steffi would be more intuitive.
Stevie as a full name? No. The only way to escape it and form your own identity would be
Eve or
Evie, which would cause even more confusion