Experienced this problem when we updated our server to 1.10. Going into the Nether was fine, but coming back it would teleport us into the Overworld with extreme Y variation (from Y = >3000 to Y = <0), which was odd since most of our portals are at Y = 119 in the Nether hub, and at first I thought that the game was multiplying the Y value by 8 in order to translate Nether to Overworld coords, but this didn't seem to happen consistently. We ended up reverting back to 1.9.4 and we now no longer experience these problems. We'll be waiting until 1.10.1, I guess.
Ryan Hetchler
Experienced this problem when we updated our server to 1.10. Going into the Nether was fine, but coming back it would teleport us into the Overworld with extreme Y variation (from Y = >3000 to Y = <0), which was odd since most of our portals are at Y = 119 in the Nether hub, and at first I thought that the game was multiplying the Y value by 8 in order to translate Nether to Overworld coords, but this didn't seem to happen consistently. We ended up reverting back to 1.9.4 and we now no longer experience these problems. We'll be waiting until 1.10.1, I guess.