Im sure the artist forgot
Its strange tho, first she's talking with him all right, and then suddenly she can't hear him? Does the author have selective memory or smth
Reading lips is a thing yaknow, shes either signing or mouthing the words at him and he is doing the same
They are using sign language,it's just the author not showing it in full depth but their hand are definitely not still.
Also when they use sign language,they are using "...." in speech bubbles
Guys, how is she talking with her father, if she's deaf??? It doesn't look like they use sign language