Background collision bug?

Hello everyone, I was just playing around with the demo of MMF2 to see if I still remember anything from the days I owned CnC, which was many years ago. Now I've found something quite peculiar as I tried to create something very simple. Here is what I did.

1. I created a gradient quick backdrop object, set its collision to "obstacle" and put some of it around the playfield.
2. I created two active objects, assigned platformer movement to one and a bounce movement to the other.
3. In the event editor I've told both of my active objects to stop if they collide with any background object or leave the playfield.
4. Starting the application with this very simple set of objects and events I noticed that neither of the two objects stopped when colliding with the background objects, but both stopped when colliding with the edges of the playfield.

I then tried to locate the problem. When converted into a normal background object, collision was just fine. Then I looked around all the available settings to see what could be wrong, and then I found that if I manually set every quick background object to the front, collision would work, but if they were in the back (just in the back, not behind anything, mind you), collision would not work, and collision wouldn't work by default either.

I wonder if this is a bug or a feature I am unaware of.