I don't know if its broken but I encounted the same issue on the site this weekend, changing my flight from AMS-LHR-YYZ to another date was excessively expensive for most dates I wanted. I assumed this was because of the pretty good price I have on the orignal flight (€ 740 for a week LHR-YYZ in PE) and the higher prices that are asked now. But I had change fares of over 2500 euro.