Same here. I did a very extensive bug report on it a few days ago.
This is very important, since eliminating collisions allows island makers to:
(A) improve fps
(B) improve bot behavior when replaced with fewer simpler collisions
(C) eliminate minimap bugs when props are used as distant decorations.
Not being able to remove collisions means that some very nice looking props are unusable for many situations.
In my case I was trying to set them to ‘no collision.’