Simple. The interest is for 2025 and comes first. The BHS hike is for 2026 and comes second.
For those under age 65 the 2025 Basic Healthcare Sum is $75,500, and the 2026 Basic Healthcare Sum will be $79,000 (+$3,500). So if your MA is $75,500 on December 31 (today — specifically tonight, just as the clock strikes midnight) the MA interest for 2025 will spill over, and then from January 1 you'll have $3,500 of room below the new Basic Healthcare Sum. Assuming again you're under age 65. Once you reach age 65 your BHS is fixed for life. (Although like everyone with a MediSave account you still open up room below your BHS whenever there's a deduction from MA.)
Interest on SA, RA, and OA stays in those respective accounts. All bonus interest lands in SA/RA no matter how you qualify for it. MA interest only stays in MA to the extent your MA is below your BHS.