Is Ch. 19 missing or is it some typo?

I was wondering the same thing!

Notes from September Scanlations: “Wait!” I hear you cry. “Chapter 20?? But the last chapter was 18b… What happened to chapter 19?” Well…we’d like to know that as well o_O The last installment of HY was in December 2015 (the January 2016 Gush issue) and was clearly labeled “Chapter 18, second half” on the cover of the chapter…and then after a short hiatus, she’s back again this issue with CHAPTER 20. What happened to 19? Is it lost to the ages? Will she realize shortly that oops she skipped a whole chapter? WHO KNOWS. For now, idk. Use your imaginations to decide what zany shenanigans Aki and Ueda got up to in the lost Chapter 19.