Ideas
No cc or mods (ever), Win 11, DX 12, memory boost disabled, still occurs on the new save I created after 12/2 update. This happens in my game but they only "travel" to the custom venue. They are not on a getaway. They look as though they are appearing in a "buddy" manner (by 2's) & on the same slot/space as well. Many merged sims like this are really funny. IMO I will vote up on this because it does occur when not on a getaway. Screenshots are on a custom venue I created.
This occurs at Camp Gibbi Gibbi too. Eight (8) children (all NPC's) and the family I use to "travel" to that location will all appear as though everyone is just arriving and about half of the arriving sims are "blended" just like the screenshots below.
I can't say anything about it occurring at the onset of or during an getaway because I have not yet figured out how to get everyone in any household involved in a getaway! It always leaves one (1) household member out.