I believe this to be a bug but it may be on purpose. It just seems like it shouldn't work the way it does. When you merge a creature stack its ability power goes off. If you merge some more creatures on it its power goes off again and if you stack another the power goes off again. However, when you do it in one move to a big creature it doesn't go off. See example layout below:
G = A big green creature already merged
y = A yellow creature
g = Small green creatures
As it is listed above nothing is happening. If you remove the yellow creature the first two green creatures merge into the bigger creature and the big creature's ability goes off. Then the second two green creatures merge and the ability DOES NOT go off. Seems like an error.