So, I found a bug with Nether-Portals.

If you build a portal and the destination in the Nether would be inside a "mountain" with no space from top to bottom of the Nether, the portal generates at a near open space.

Now to the actual Bug.

If you leave the Nether to this generated portal you generate a new one in the Overworld at matching coordinates (wich is nothing new) BUT the wold does't load propperly (Mobs stuck where they are, you cant pickup Items, only 3x3 Chunks are "loaded") and you need to exit and open this world again. after that rejoin everything is normal.

Only mod used: Optifine_HD_G9_pre17