Zbrush | Why hidden open edge can prevent Mesh Fusion from happening

Another observation I have is the proximity of open edges. If you have a hidden open edge nearby the newly applied IMM Insert will fuse with it. For example, I have half of the sphere hidden, not deleted, just hidden. The newly applied IMM Insert will fuse with it even though I have polygrouped a star just below it.

In order to fuse with the star shape below the hidden open edge of the sphere,

along with the subdivision levels, in order to make sure that my IMM Insert brush fuse with the star shape below it