User blog comment:B214/Question and Answers 3/@comment-79.130.78.230-20140719120320/@comment-24502975-20140719170323

Well, volume 19 would most likely be released late October. March 2015 is a short story compilation volume, bundled with a season 2 OVA. Since that's only 5 months apart I doubt they would release volume 20 in February and then release a short story volume only a month later.

Then again I could be completely incorrect and the short story volume isn't part of the main story at all.