Sorry to revive a dead post but just wanted to share a solution I found for anyone else having this issue who winds up here after a google search.
Through trouble-shooting I learned that the issue, for me, seemed to be caused by clipping my blacks.
If I use
MONONODES "Clamp" DCTL on a node right before the guassian blur node, the issue goes away completely. This basically prevents any black values from going below 0 and any white values from going over 1. That prevents the gaussian blur node from freaking out and adding rectangles for whatever reason.
Hope this helps!