Tried with the last Forge releases for Minecraft 1.10.2 and 1.11.2 ( Forge-12.18.3.2221 and Forge-13.20.0.2226 respectively).
After death, you always respawn on negative X and Z coordinates (always northwest of the world spawn).
How to reproduce the bug:
1. Start a new game.
2. Type the command /gamerule spawnRadius 5000 (for example).
3. Kill your character.
4. You respawn northwest of the world spawn on negative X and Z coordinates.
Probably this has something to do with this issue:
https://github.com/MinecraftForge/MinecraftForge/issues/2624