I’ve had the iOS password app think that it changed my password, when it did not, and then lose my old password.
It doesn't retain all previous passwords??? that's crazy.
It does now but didn’t always: https://austinmacworks.com/updated-passwords-app-adds-histor...
I should also add this is only if you have iOS 26 or newer.
It retains past passwords. Open the entry in the Passwords app and then scroll down to "View History" button.