unexected behaviour in gltf export

Dear experts,
I found an unexpected behaviour in the gltf export. For illustration the following script to be executed once with exportType='step' and once with exportType='gltf' will create the files test.step and test.gltf respectively.

```
import cadquery as cq
exportType='step' #'gltf'
pts=[[19.5,0,0],[9.5,0,0]]
assy = cq.Assembly()
w = cq.Workplane('YZ')
tube = w.rect(1,1).rect(0.95,0.95).extrude(-9)
for p in pts:
assy.add(tube, loc=cq.Location(cq.Vector(p)))

assy.save("./test.%s" % exportType.lower(), exportType=exportType.upper())
```

If I open now the file test.step in FreeCAD I find two tubes one after the other - my expected behaviour:

If I open the file test.gltf in Blender, I can find only one tube and an error message - opening the file in three.js is not possible at all:

To me this looks unexpected and I suspect a bug as other formats like STL are working like test.step.

Many Thanks for any insights!