For Australian coins at least, it's all about the legislation.
The new Charles III royal portrait for Australian coins was not authorized until October 2023. Coins already legislated for up to that time in a sense already have their designs "locked in"; once the legislation is approved, the Mint normally starts work on them immediately, if they hadn't preemptively already started.
Sure, they could cancel the legislation, and scrap all the work already done on preparation and production of coins, but that would put everything behind schedule.
I believe the Mint announced, way back when the Charles III portrait was announced, that Queen Elizabeth memorial portrait coinage issues would continue up until May 2024 - that's how far in advance the release dates for upcoming coinage issues had already been set for coinage already legislated when the new portrait was announced.
It's after May now, so anything issued from now on should all be Charles III coins.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis