Another data point that the splitting at T-24/checkin now preserves companion benefits.

2 pax itin, 1K/GM, on roundtrip with stop each way. CPU for both on first leg at T-96. Split at checkin on app, with notification (split or no upgrade). On return, companion had 1K benefits, including 3 bags and group 1. Showed on waitlist below me (although despite consecutive sequence numbers was several names below on one of return legs - presumably a bunch of others in same fare class).