Your 2025 MAGI affects your 2027 IRMAA, not your 2026 IRMAA.
As VolvoGirl said, you can split your 2025 RMD between 2025 and early 2026. There is nothing special about reporting RMDs (as long as you complete the RMD by the deadline). The RMDs are reported just like any other distribution. The IRS doesn't care that the amount that you will be reporting on your 2025 tax return is less than the full RMD for 2025. No explanation to the IRS is needed.
