While I don't think it was ever explained, it was intentional.She was the only fangster that cast a reflection, and it happens again, too.
One possible explanation is....she was a supernatural being,
And all other vamps were human, then bit by something.
She was never bitten.
Just an often-deceased witch that got transformed by Nicholas, a lot.
There WAS a Magic Mirror scene where they tried to keep her out of the shot, but still caught her hair in the mirror. If that was the first time......maybe the writers decided to make her the exception to Vamp Rules.
Later, in 1897, she uses a mirror to capture Barnabas' vamp image, and create a doppelgänger, though Barnabas never sees that image...another seeming violation of The Rules.
A great scene in the Johnny Depp movie.....brushing his teeth in a mirror....all you see in the mirror is the toothbrush and the toothpaste on fanged teeth!