It's actually never been uncommon for Roundabout and Manhattan Theatre Club to announce their seasons piecemeal. Roundabout has announced one Broadway production (Time and the Conways) and MTC has announced two (The Prince of Broadway and The Children).
Looking at announcements for last season to put it in context: Based on press release dates, Roundabout announced it was producing The Cherry Orchard and Love Love Love on November 18, 2015. The Price was announced May 26, 2016. Marvin's Room was announced September 6, 2016, and If I Forget on September 12, 2016.
So although some theaters (like Signature or CSC) announce their entire season at once, it is not uncommon for others to announce shows one at a time.