I investigated this further today. This is an attemt to describe the problem and a "solution" to it.
In AC22 I created a new gdl object and typed BRICK A,B,ZZYZX in the 3d script. Then I placed a few of them and used slabs to subtract shapes. Next step was to save it as FBX and open the file in a FBX-viewer. The result was a damaged version of the geometry.
Now the weird solution:
If you add a dummy array parameter (of any type) to the parameter list, the export works fine. This took a while to find out…