There's a really annoying bug with monstermakers with Spirit 1.8a1. Spent a long time trying to figure out why monsters often would spawn a random distance away from the position of the monstermaker.
Looked at the code, turns out there's a missing else statement which causes the spawned monster to always inherit the velocity of the activating entity, so if you run into a trigger_once with some speed the monster will come flying out of the monstmaker with the same speed, landing some distance away.
A feature of Spirit 1.8 monstermaker is the velocity of monster keyvalue, so I tried setting it to 0 0 0. Doesn't work anyway: turns out that's another bug: the targetname of an entity to copy the velocity from should be set and not actually a vector as is stated in the fgd.
There's a workaround, if anyone ever has the same problem. In the velocity of monster field, enter the targetname of an entity you know will always be stationary. I used a path_corner.