Are you, by chance, playing with a gamepad and only highlighting the monster in question, not fully switching your target to it? The enmity list in the party menu shows the enmity for your current fully-selected target, not a target that's only highlighted (even though it blinks as if it's switching targets to display).

Whether that's a bug or not is up to SE, I guess; I think it ought to be changed. But if that's what you're doing, that's why you're seeing what your seeing. If not, weird, and definitely a bug.