New code written today will use the new version and have the correct behavior from day 1.
Old code that is maintained will eventually be upgraded, which yes does come with work sometimes where you realize your code works on version X but not version X+10 and you do a combination of tests and reading patch notes to see what changed.
There is no "correct" behavior here; either one is a valid choice that can be documented and that programs can rely on and exploit.
Code doesn't care about when it's written, only what you run it on, and with what compatibility options.
E.g. one possibility is that ten-year-old code that wrongly assumed the opposite behavior, and has a bug, will start to work correctly on the altered implementation.
Old code that is maintained will eventually be upgraded, which yes does come with work sometimes where you realize your code works on version X but not version X+10 and you do a combination of tests and reading patch notes to see what changed.