I think I reported this when the gooey mass mega-spell was first introduced.. I haven't used it in a long while but I've had some games with it recently and I think there is still a bit of an anomaly with the blob spread physics (regarding the placement of the three target zones).
The issue is how it calculates what happens with each of the 3 targets that you place. If the hex's which the blobs spread to overlap upon each other they tend to just cancel each other out (you lose a blob or two).. This means each target you place does not guarantee 4 blobs each (like the base spell does).
You can place the targets perfectly to cause the blobs to spread to the only 4 hex's that are available (for each cast). However because it is a triple cast it doesn't factor in where the other blobs (from the other two targets you placed) are going to spread to.
If the '4 blobs per cast' rule was applied to gooey mass (1 blob on target hex + 3 random spreading blobs to available hex's), some of the blobs that get cancelled out by other blobs would perhaps spawn or spread to the next free hex surrounding the target hex (thus spreading to the hex it should have spawned on anyway).