At least, I think this is a bug? It seems like one cause I don't recall the Wither firing at Armor Stands, nor do I see a reason why it would since they're not passive/neutral/hostile entities, they're just... Armor stands.

Problem:
The Wither's extra heads also sometimes focus the Armor Stand even when the player has hurt the Wither, which can be used as an exploit if you make enough stands or somehow hide it to where the Wither cannot reach it. (Likely in Obsidian)

 Note:
Apparently wiki's are stating the Wither ignores undead mobs, however when I summoned some of them, the Wither immediately began shooting them. Not sure if the wikis are just incorrect or this is also a bug. (even if added to a friendlyfire false team)

Again would love to provide screenshots but this Jira thing is really bad. Keeps saying my screenshots are invalid and missing a "token" whatever that means. Really to replicate just spawn in an armor stand and a wither, it'll go for it.