Mine is a little bit different scenario, but similar.
When a holy freeze mercenary goes to the Normal-difficulty game, the aura is correct (which is the holy freeze), but the skill-icon on the mercenary panel shows Defiance.
That’s correct aura, since you hired it in nightmare. The tooltip changes according to difficulty for some reason, but the aura remains the same.
It doesn’t always remain the same,
sometimes it does, sometimes the original aura is just not there.
Lemme remind you that blizzard is a young startup full of life.
And taking on Diablo II is a huge challenge for them.
Just be patient guys.
May I suggest you guys at blizzard get your hand on The good old “Kernighan & Ritchie” and seek for the chapter dedicated to act 2 mercenary implementation 
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Haven’t seen it change once, merc is always using the same aura.
This is happening to me. I hired a Act 2 Normal difficulty Merc with Defiance, but now that I am playing in Nightmare difficulty, the same Merc is showing up as “Holy Freeze”. Blizzard please fix this!
I still need to check if it’s just a display icon bug, and that the Defiance aura still activates or not.
Update: Okay I just confirmed, even if the icon says “Holy Freeze”, my Merc still activates his original (Defiance) aura. I guess it’s just a visual bug, but works as expected.
It’s not often but sometimes you’ll see the aura is just not here at all.
And you get zero regen, be careful and you’ll spot it happening at some point.
However that is probably unrelated with the icon bug and is just another issue.
The game also crash sometimes I don’t remember Diablo 2 crashing so much.
That must be the resurrection, the brain might be damaged…
Maybe this is undead diablo 2 
omg can you trust the game ?
Maybe it’s gona bite you in the middle of the night ?
Any update on this being fixed? Same issue Merc was hired in Nightmare ( Might ) and displays Blessed aim when in hell. This is rather annoying.
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You are wrong. It’s not visual.
Tested it in Hell with a Holy Freeze merc from Nightmare.
For some weird reason it took super long for the aura to activate, but even tho it says Defiance, it was in fact Holy Freeze with monster getting cold.
I gain no bonus Defense stats either.
Everytime i hop back on Normal difficulty the Merc displays defiance, but the second i hop back on Nightmare he displays Holy Freeze and casts Holy Freeze.
And even if he somehow manages to keep the Defiance when i get on nightmare, if he dies and i have to revive him. It’ll revive the merc with Holy Freeze.
Typical Blizzard, can’t seem to release games without everything bricking and falling apart.
Works fine for me, merc never changes his aura, only the tooltip changes when switched to another difficulty.
I had a merc with prayer and went to NM and it is now thorns. I noticed when I was not healing. So it is not just visual or saying the wrong aura. Not sure if thorns is even working but I know prayer is not happening.
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Definitely having the same issue as you, Yumeria. My mercenary changed skills going from normal to nightmare, and from nightmare to hell. I didn’t realize it at first because I didn’t expect it to happen. But then after playing for about 5 minutes, I began to notice that I wasn’t as protected as I once was. That’s when I realized my hirelings abilities had changed, and couldn’t understand how or what happened to cause that.
I got here since I just noticed the same Azrael with Holy Freeze I’ve had since second 1 of Act 2 NM is using…nothing…I mean…it now says Defiance in Hell…except there’s no glow for any aura from him.
3 Countess runs ago he did have the holy freeze blue glow around him.
Currently with no glow there is zero freezing going on. I was wondering why I’m dying and/or suddenly getting ran down by axe wielding maniacs.
Edit: seems to have…started back up after 20 seconds+ of jabbing some yeti (all of this was happening outside towns btw) the blue glow is back…it’s Holy Freeze but tooltip says Defiance still. Schrödinger’s Aura.
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I equipped my hireling with insight, when I hired a Hell act 2 merc with Defiance the aura and added defense bonus would often times not activate. I thought maybe it was a big due to insight, but no it actually doesn’t activate all the time which is dumb. I do many MF runs and the aura at times never appears.
Also just like everyone else, my NM Act 2 Holy Freeze Waheed is now showing as Defiance, however neither aura actually show up. Weird part is that holy Freeze was working at one point, just stopped randomly and now only active aura is from insight.
Your merc has a chance to activate their aura with every hit that they make. They need to do some poking first.
Same issue. I hired Fazel with defiance aura in normal and now it says the freezing one.
I have the same problem here! I also find it odd that this thread has a reasonable amount of attention but doesn’t show up here in the forums. Google helped me find this topic. 
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I have an additional issue with the bug i dont know other people have it. My merc will show defiance as the aura but freeze the enemies with his holy freeze. When i make a new room he changes back to defiance aura. Everytime i make a new room his aura swaps between holy freeze and defiance.
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I think it’s a bug with the merc’s skill icon and tooltip, but not the skill itself. Here’s a screenshot of my NM merc with Holy Freeze in Normal difficulty. He has the HF aura and we can see mobs getting hit with its pulse, but the skill icon and tooltip show Defiance.
https://imgur.com/a/VWlkg30
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