Is a shape after offset sometimes converted from a solid to a shell?

[raydeleu]

I wanted to create a holder for a GNS receiver using the makeOffset function. The idea was to create a model of the receiver first, then create an offset to this shape and cut the upper part. This would look like this.

However,the receiver has rounded edges. As soon as I apply the filleting to the shape of the receiver, the following happens:

My example code is listed below. To witness the effect the line starting with ".fillet" should be changed into a comment :

const defaultParams = {             // setting the value of the parameters
    length:   100,   
    width:    50,    
    height:   12,                     
    thickness: 2,
    portion:   0.75
    }
  
    function main(
   { makeBaseBox, Sketcher, makeOffset },   // functions used within the main program
   { length, width, height, thickness, portion} )  // parameters to adjust the model
  {
    
    let receiverBody = makeBaseBox(length,width,height)
    .fillet(5,(e)=>e.inDirection("X"));
    let cutter = makeBaseBox(length+4*thickness, width+4*thickness, height)
    .translate(0,0,portion*height)
    let holder = makeOffset(receiverBody,thickness)
    holder = holder.cut(cutter)

    let shapeArray = [
        {shape: receiverBody, name:"receiver", color:"red" },
        {shape: cutter, name:"cutter", opacity: 0.5},
        {shape: holder, name:"holder", opacity: 1.0},
    ]   

    return  shapeArray
   }

I found a way around this by not using the makeOffset function, but still would like to know why this does not work.