The 24 hour rule is a bit
I've even had to convince phone agents that I have already applied a SWU to a reservation (ORD-EWR-FRA), so that they'd clear the SWU for the domestic portion (when R opened) when the international segment had already cleared. This has happened twice within the 24 hour window and they tell me I can't apply the SWU. After they see it's already been applied then it's no big deal as they do not have to reissue.