There has been prior discussion on this topic. Perhaps the Search feature will help you. There was also a Q&A in a prior Gray Book, and a verbal Q&A at a prior Enrolled Actuaries meeting. The IRS is sympathetic to such timing issues, so you can use 12/31/24 as the final date for Plan B for all purposes.
Note that one second after midnight on 12/31/24, the Plan B assets are owned by Plan A, no matter where those assets are located; the "received" date is not relevant.