Why does the bottom drop out when stacking these shapes?

[edgy ridge]

3D viewer links :
{RbRb----:RuRuRbRb:RbRbRuRu} {RbRbRbRb:RuRu----:RbRbRuRu}

[dusky nebula]

Were you trying to combine a half shape by stacking it?

[sterile sinew]

the fact that they deleted their message probably means that they've figured it out 😅