I tried to research some fares on Singapore air.com for next year and the calendar shows dates up until the 3rd of April 2020, so roughly 355 days ahead which is how it has always been. However, when flexible search is chosen the last day you can possibly book with all flights available on a given route is the 28th of March and already one day later only limited flights are showing up. After that the search engine returns an error for any other day.

It looks to be a glitch but I hope they will fix it soon. Otherwise they basically are limiting advance purchase to around 348 days now which makes little sense as the calendar still shows the full 355 days.