Thanks! Whilst this wasn’t the problem, it lead me down the path (a long path of trial and error) to figuring out just what the bug actually is: It has to do with the new ability we have to sit our followers on chairs.
Long story short: If you put your follower on a chair to sit, this is what causes the bug that’s apparent in video above I linked. This is why most of my followers were buggy; I had them sitting around campfires keeping nice and warm when not following me.
To stop the bug, simply put them on guard standing somewhere, then log out and reset your server. The next time you have them follow you they will once again fight as they should in battle.
Want to recreate the bug? Simply put them sitting on a seat to guard. I tried this method with six of my followers (two fighters; Lian and Janos, three Archers; Freya, Dina, and Archer Dennu, and Cimmerian Berserker) All we able to bug and not-bug according to how I had them guard (on a seat or standing).
Hopefully a developer/someone-from-Funcom sees this and can fix it for future.