My healer merc casts Symbol of Sanguineous and then casts Persistence which overwrites the previous buff. Not game breaking, but kinda dumb. Level 120 and full aas.
Always been true and I think not a bug, but a design choice. It's intended to play nice with the usual preference of symbol+skin+ac, except for the few classes that want Aego (paladin, cleric). I see the problem more as healer merc doesn't cast Holy Armor line (e.g. Ward of the Guileless) to fulfill the AC for the first choice. Just block single target symbol and you're good. ps: Merc never had cast group symbol either.
It's always been this way. This is how they work. Some like Symbol and Skin, so they cast those related buffs first, since Persistence can overwrite. Thus, you need to either Block Persistence, or block Symbol & Skin.
