I like to think these are two separate characters
Mater Lachrymarum, the secundary antagonist in Dario Argento movie "Inferno" and main antagonist in it's sequel "The Mother of Tears" was portrayed by two different actresses in the two films, being Ania Pieroni in Inferno and Moran Atias on TMoT.
And while in theory both are meant to be the same characters because Ania Pieroni couldn't reprise her role for the final movie in the trilogy, I have the theory of the change of actress being something that actually has an in universe explanation.
Because besides of the different look, the character portrayed by Ania is fundamentally different to the one portrayed by Moran in the 3rd film.
"The Musical Student" as is named in Inferno, is a quiet and almost esoteric presence, a mysterious woman which intentions aren't directly revealed to the audience. She stared and observes, always accompanied by her cat, as she stalks the main character without taking direct action.
"Mater Lachrymarum" as she presents herself, on the other hand is a very sadistic and evil witch much more cruel and agressive than we previously seen. Her personality takes a sudden shift and is unrecognizable from her previous appearence.
But here's the thing, what if both of them aren't the same person and "the musical student" is the *true* Mater Lachrymarum?
Keep in mind, Mater is a title for an all powerful an ancient which, but taking inspiration from the Suspiria remake, it woulnd't be crazy to think any which could claim to be a Mater due her powers, despite this not being necesssrily the case at all (spoiler for the Suspiria remake if you haven't seen it)
And to this there's an important idea to keep in mind: The Mother of Tears isn't the vision of Daria Nicolodi, the original writer of Suspiria/Inferno.
And to which Daria herself said this: "The real third mother isn't really dead"
So in theory is interesting to imagine that the musical student played by Ania Pieroni is secrectly the real Mater Lachrymarum who stayed offscreen all this time, while the witch played by Moran Atias who was killed in TMoT by Sarah was just taken her name. And as such the evil still continues existing in the world.
It's funny to picture how after all the craziness of the Mother of Tears, the main characters celebrate thinking all is over and the witches are finally all dead, while Mater Lachrymarum is probably watching while she pets her cat, knowing her moment is about to come.