-12341 is the value of the NONE constant (one of the reasons I picked that number was to make it instantly recognizable). You need to set global.load_room to the current room before changing to the evolution room (and also the global.load_x, load_y and load_direction variables to the player's current x / y / drawdir respectively) otherwise it doesn't know where to put you back - and that's exactly what's happening right now.
(For an example of how to do this, check out player_step_fill which handles the "steps in tall grass to the next random battle" counter, and which sets these before changing to the battle room if an encounter is triggered - side note, this function would also be where you put things like "monsters in daycare gets EXP" and "countdown to eggs hatching" happening)