Same here. I was calculating some totals in a report written by a colleague and kept getting a result that was $.02 off. I thought her calculations were wrong, so I asked her to double check her work. Turns out, it was my work iPhone which had recently been updated. I added the amounts using an old handheld calculator and confirmed that the total summed by the iPhone running iOS 18 was incorrect. My personal phone, which is still running iOS 17.6.1 calculates the correct totals, so it's definitely linked to the iOS18 update.
Phone running iOS18: $110,765.52+$185+$60,000+$71,000.95+$20,018.35=$261,969.80
Phone running iOS17.6.1: $110,765.52+$185+$60,000+$71,000.95+$20,018.35=$261,969.82 (the correct amount)