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.