Sorry for the wall of text but there is A LOT to talk about !

I'm not sure what's happening but it's really weird.
I just got a big item upgrade in the beginning of act3 and I noticed something was really off when I checked the max health increase I got compared to what I was expecting, so I started investigating and ended up running in many issues, with at least one of them potentially game breaking in my opinion.
So anyway here's what happened to me when delving in that rabbit hole :

1. My first though was that items were giving too much health. I currently have 3x10% and 1x8% max health nodes in my skill tree. If I look at my pants, they give me 113 health, which should translate to a 156 health bonus with the talents increase. But when I equip them, they give me 202 health. That's a ~79% increase.
   Same thing for my gloves. I have one that has 26 health. That should be a 36 health bonus but when I equip it, I get 47, which is a ~81% increase.
   Last example, I use a catalyst with a +100 health gem socketed in it. When I equip it, I get 179hp.
   There really seem to be a ~80% systematic increase, rather that the 38% I expect.

2. I was wondering if maybe toughness was involved in a way. I don't know how it really works but I was wondering if it was a % increase for all max health sources, and if that was the case it could explain that 80% found earlier, but it doesn't add up. I have 54 toughness so if each toughness point was a 1% increase, the total % increase would be 38+54 = 92.

3. I started wondering, what if its not 1% per point but something like 0.XX something. So I unequipped my only item with toughness (a +20 all attributes amulet) to see how my max health would change. Well... it didn't. I went from 54 to 34 in my stats panel, but my max health wouldn't move (the force shield did)

4. As I started to suspect snapshotting issues, I stripped my character naked to make better tests (I also exited the game and relaunched it, just to be sure).
   So, my stats naked are 1854 health, 140 force shield and 34 toughness.
   If I equip my +20 all attributes amulet, the 4 attributes go up by +20 as they should (I'm sitting at 54 toughness) but it has absolutely no effect on my max health (force shield goes up to 145 though).
   I had a thought so I bought a random toughness amulet (only gives +24 toughness) to see if there is a bug with the all attributes stat, and it seems to be so because that +24 toughness amulet managed to change my max health from 1854 to 2058 (force shield went up to 146)

5. Now that I had an item I could rely on to change my toughness value, I picked up the test I was talking about in 3. So, while being naked with the stats in mentioned in 4., I checked again the value for the 3 items I mentioned in 1. and that time I got different results. The health increase seemed to be 63%.
   I then equipped my new +24 toughness amulet and conducted the same exact test and got a 81%. Now I had a pretty strong proof that toughness was indeed a +1% global increase on all max health source, but the thing was... it still didn't add up with the +38% max health my talent tree is giving me (38+34 != 63)
   I'm still not sure why is that, but I would guess that the talent tree nodes might be bugged and give less max health % than intended (sorry, I don't want to spend the respec cost just to try this, I'll let you guys do it internally :p)

6. I was still worried about snapshotting issues so I kept messing around with my items and I found a way to get the +20 all attributes amulet to work.
   I had the idea to buy another random blue item with toughness on it (and no max health so it didn't mess with my tests). So I got gloves with a +11 toughness bonus.
   When I equip them, I go from 1854 to 1947 max health.
   If I equip the +20 all attributes amulet with the gloves still on, nothing changes. But if I then unequip my gloves, my max health gets finally updated. (note, this also works by equipping the gloves after the amulet)
   I'm now sitting at 2024 max health with only my +20 all attributes amulet equipped, which seems to be the expected value.

7. Problem is, the snapshotting also works the other way around, and this is where it gets really scary in terms of in game abuses.
   If I unequip my +20 all attributes amulet, my health is once again not moving. I now have the max health equivalent of 20 toughness that I shouldn't have, since the item is not equipped.
   And the scary part is that this value does not get updated until I equip (or unequip) an item with a +X toughness or a flat max health bonus.
   What this means is that I could equip another amulet with very good stats but no max health nor toughness, and still retain the max health from the all attributes bonus of the first one.
   I don't know how high you can get with those omnistar amulets but I got a +30 one at lvl 20 in act1 with my offline character when the servers were down, so I can only imagine the massive boost it could be for an endgame item.

8. Does it stop there though ? Of course not :p (sorry)
   What ticked me off in the beginning was that my big item drop was giving me LESS health than intended. Not more. So I went full circle and came back to that.
   I'll spare you all the details, because this post is already way too long and I just want to go back to play, but I found out that +max health gems are also bugged. If you equip an item with a +max health gem socketed in it, any base max health bonus on the item (other than the gem itself) will be ignored until -you guessed it- you actualize the toughness/max health value of your character by equipping (or unequipping) another item.
   It's actually worse than it looks because until you actualize you max health, the base health value of every single item you have (except bonus from gems, but including max health affixes) will be nullified. Taking a portal doesn't even change anything. You have to gearswap something to directly change your max health. No wonder the boss at the end of act 2 could global me so easily. I was running around basically naked ! (health wise at least)

I have not found a way to exploit this the other way around, but it really is worrying considering what can be done whith the omnistar amulet. If there are some items (other than amulets) giving all attributes, it could without a doubt be exploited in the same way, and have characters with absurd amount of health while having many items without any max health on them.

Well, that's it for me. Good luck sorting that out. I hope it will be hotfixed quickly.