1/ Stacked islands aren't packed if they're out of the UDIM grid. When Stacked, island(s) should be packed normally as if they were single islands. We should be able to single click on a stack and move it instead of clicking and dragging a selection over the stack to be sure we get all the islands when moving them manually.
2/ The pack algorithm takes stacked islands into account only if they are inside the main UDIM. But then the packing computes an unusable result where islands overlap (if I select only one island in every stack it gives a perfect result). Why aren't stacked islands of perfectly similar triangulation treated as single islands ?
Anyway, I'm stuck having to work like old times because stacks don't behave as they should : importing in RizomUV only the halves of each object and a single unit of repeated objects (screws, etc), unwrap/pack, then rebuild the mesh later in 3DS Max or Maya, with missing halves and instances.
If I missed anything and someone has an idea, it'd be great. And if it is a glitch, then please consider checking on that. That's a serious handicap given how great the stacking tools are. Thanks.