Why a buff is replaced, rejected or removed when another is cast.
Replacement uses the recorded abnormal effect group and abnormal level, not merely the displayed skill name or a shared stat. In a conflicting group, an equal or stronger incoming effect replaces the previous one; a weaker one is rejected. Instant herb effects can temporarily suppress an ordinary buff while its original timer continues.
Ordinary buffs and songs/dances share the total buff limit here; dances also have their own configured cap. At capacity, the oldest eligible effect can be removed. Toggles, debuffs, Seven Signs effects and healing potions have special handling. Check the exact skill and settings before calling a disappearing buff a bug.