Why the “difference boolean” (object red - blue object) fails?
The blue object is a group (5-cylinder). The boolean operation is not performed between a group and a simple volume?
To function must perform the boolean for each individual cylinder.
Probably because the blue cylinders don’t overlap nicely at the top. What I did in the file below was to draw a box around the top of the cylinders and the BooleanUnion command on the box and the cylinders. Then, I used the result of that to subtract from the red volume. That works as demonstrated in the file
Exactly. Like I said, you can’t BooleanUnion the cylinders because they don’t overlap nicely. Therefore, a combined boolean difference is unlikely to work.