I think you mean affect, not effect. Affect is an action in progress, effect is what is left after.
I’m not sure what you mean by your question though.
Are you thinking Nagini should not be able to take human form?

Wait for my daughter to come home from uni, she’s an absolute HP genius & will no doubt blow this out the water as usual.

Meanwhile I will re-watch the movies. I have Fantastic Beasts, but I don’t remember any reference to Nagini.
You said new Fantastic Beasts trailer. Is there a second one?

My pardon for neglecting a link:

Here is the trailor

Yeah my daughter has said she’s not quite sure how they’re going to explain that, because if she’s permanently a snake now due to being a maledictus then how on earth does she transfigure into Bathilda? Hopefully it will be explained when all five of the Fantastic Beasts films are out.

Nagini was a horcrux.
Could it be possible Voldemort was able to cause the transfiguration?

From what little I’ve read the maledictus at some point is forever changed into the animal they are destined for, In the trailer she works in a circus and changes from human to snake. Not sure what causes that final transformation to happen and remain. That might explain it.

Here’s the scoop.
My daughter knows the whole wizardry scene in an out. According to the book, Nagini did not take Bathilda’s shape, she killed Bathilda, and manipulated the corpse.