It is possible that I'm having also the same issue, but I can't say it 100%, since I'm using Maxwell's export alongside a Master script.
It is the first time I've encountered this, however: in LW11.5, not in earlier versions. The problem is that some meshes will fail to export.
I've tried fiddling about with DynamicUpdate 0, thinking that it might be something to do with the mesh refreshing in Layout, but I can't really say if it helped. It is totally random; sometimes, after exporting repeatedly, it just fixes itself and has no problems.