I don’t know whether this is a bug or just a quirk of magic night. I use both Lady Locke and Sartana in my attack, both have damage over time effects, but they seem to operate differently on magic night stages. Locke’s concentrates all her damage into 1 move, whereas Sartana only does 1 turns worth of damage (1/6th of her total). Hopefully these images explain it better. Needless to say means Lady Locke is brilliant on magic night stages.
EDIT: On 23-Feb-2020 I noticed the Lady Locke damage on magic night has been changed in one of the updates, she now does the damage of her fourth turn.