przemratajczak/mangos — comment on commit 11a75b5f0e6d39b2d277a365bf9b6511d9bc8d8e
File: src/game/SpellEffects.cpp line 10564
By rsa:

fz > VMAP_INVALID_HEIGHT_VALUE always. in this case your may teleport on other floor of instance, or under vmap surface...