Is it scientifically correct to derive conclusions unrelated to hypothesis from A/B test data
It looks analagous to drug testing, where reporting of side effects during drug trials is obviously very important - i.e. the increase in Y seems analagous to a side effect. And some famous drugs have begun their lives as research into a side effect. Viagra is probably the most famous case, being a spinoff from a drug developed as angina medication. So in your write-up on your experiment you should definitely report the apparent effect on Y.
However, if the effect on Y is commercially important, then you still need to go back and do an experiment around a hypothesis that references the increase in Y to validate the existence of the effect properly.