As you can see in this thread old followers indeed do have a problem.
Previously their armor was calculated from armor items + agility contribution.
Agility now has been completely reworked thou and no longer provides any armor…
Stats for existing followers thou were not changed in any way, so they’re in your database the way they were… which means they simply do not have a database record for Grit whatsoever so every follower you had will have 0 Grit - which is the new stat that contributes to armor
This means that pretty much every max level follower you placed prior to this update is missing on average 300-400 armor, on top of that even the armor damage reduction curve seems to be less steep, at least that’s the impression I’m getting, didn’t check to confirm yet.
Edit: yup, confirming the armor curve change:
this is my armor and reduction ingame now
This is the old armor curve from the wiki
Based on this I should be around ~64% instead of 49%. So apply that to the thralls too 