OK, found the original documentation of the multiple selection group gremlin. This example of copy/paste operations, but you get the same result with audio snap>split to clips. Sylvan, this is what you're getting at, right?
Step 1: create a selection group (no. 104 in this case)
Step 2: copy & paste selection group (resulting new selection group is no. 109)
Step 3: select both groups
Step 4: copy and paste both groups (result is BOTH new sets of clips are in one selection group, no. 110)
Before X3, step 4 would result in each set of clips being in its own selection group (e.g., no. 110 AND no. 111). From X3 on, this is the standard behavior, and there was never any public discussion I'm aware of about said change. It should be obvious enough why this causes headaches where there didn't used to be headaches....
And while I appreciate how much work the Bakers put into refining things and also that this isn't the place for feature requests, can we pretty PUHLEEZE at least have an option in preferences for switching between pre-X3 behavior and current behavior when operating simultaneously on multiple selection groups? Pretty please with a cherry on top?