This is done with the Nov 21 beta plus Nov 28 NMTLib.dll.

Simply put, unioning the two pieces causes some sort of weird deletion of the larger piece. For the last hour I have tried a variety of things, like changing the size of the smaller thing, moving it around, simplifying the mating faces, and so forth, to no avail.

Please help! My petite crabe de fer wants to be born in the 3D printer, and this is almost my last step.