I’m going to post this problem and the solution I encountered this morning, as it might be useful to others who see geometry oddly moving location while attempting to group it.
While cleaning up an imported .dwg I was attempting to select all instances of a “seat” and to group all the seats so I could assign them a layer and preserve them while I run CleanUp on the file. I selected one instance, selected all others via right click, so far so good. Proceed to make group and… huh? Group fails to form and some of the geometry has spontaneously moved location.
Turns out some of the seats were nested in groups of their own. Once I discovered this and exploded the smaller groups the seats were able to group correctly. I had not seen geometry move on it’s own like that and was stumped, so if you see it happening, I guess check for errant nested groups.