It is still a case of 文字化け. 文字化け means the phenomenon where characters are shown incorrectly on computers, and its cause is not necessarily a mismatch of character encodings.
I do not know a specific term for the kind of 文字化け which you are talking about. I would say something along フォントが足りないことによって起きる文字化け.
By the way, the glyphs used in this situation are technically called replacement glyphs in English. I do not know how they are called in Japanese, but I guess that there is some technical term for them in Japanese.
Update: As Dono pointed out, some people use the word 文字化け in the narrower sense which you explained in the question. (See the note page of the article in Japanese Wikipedia to see an example.) I do not know if there is a better term which everyone agrees on.