The armor models for the custom armor render a bit awkwardly when placed on things like armor stands. In all of my tests, this only effected the helmets. All helmets regardless of armor stand direction seem to point south.

I have no idea about this one 0.o

It renders fine on players, and on zombies and skeletons that can pick up loot, so not sure why armor stands act weird.

wait, I think this is a vanilla thing.
Activate F3+b and a blue line should come out of the armor stand showing where it's pointing

Yeah, it's definitely a weird issue. I noticed it in one of my test mods, and found that the issue also exists here. I am not sure how to fix it myself ':p

You're right, they do seem to be looking south. The rendering for vanilla armor sets must have some special code for them to correct the rendering, because they work as one might expect.

looks like all entities initially point south, if you put down some sparks they are facing south aswell

South is the default rotation for almost everything in the game, so it's not that surprising that it would point south unless it is told otherwise by some internal logic.

Most likely they change the rotation when the helmet is placed onto the armour stand

but I wonder why vanilla helmets don't have this issue (or do they?) maybe it's updated when a player puts a helmet on them

I have compared it against vanilla helms, they don't have this issue.

I just did a test of my armor without the special model and other rendering changes, it seems this happens with default armor as well. That is unfortunate.

This is probably a forge "new feature in vanilla" issue if it happens to your armors too, vanilla has all sorts of specialcasing around the armor stands (-.-)

Looks like vanilla definitely does some rotation on the render.

I did some tests with Thaumcraft and found that the thaumcraft armor is not affected by this issue.

I might have a look at this later on, and create a PR. I have some ideas after looking through Thaumcraft.

The chestplate armor has its arms wiggling slightly as if it was equiped by a player instead of an motionless armorstand

Pinging this to let everyone know I found a workaround, check the forge ticket above (MinecraftForge/MinecraftForge#2508) for more info