You mean that the head is covered, but the hair hanging down beneath it is visible.
There is some room for discussion about this. The biblical source for hair-covering is the Sotah, and it says "the Kohen shall uncover the woman's head" (not "her hair.")
Rabbi Yehuda Herzl Henkin thus writes in Tradition 37:3:
Nevertheless, R. Feinstein could permit more than a tefah outside
the hairline simply by following his own line of reasoning. Since the
Torah specifies uncovering the sota’s head and not her hair, the kohen
needed to uncover only the hair on the woman’s head, and not what
hung down on her neck and shoulders. Consequently, no Torah obligation can be derived for a married woman to cover hair outside her hair-
See Rabbi Henkin's article for much more on the subject.
I'm not saying it's permissible, but there is certainly room for discussion about it.