Grade 3 Module numbers

Hey. Not sure if it was just a typo, if thereā€™s something planned or if Iā€™m just thinking way too hard here lol.

For the Grade 3 lessons, there were planned modules, 15-23. However, after module 21, the immediate next one is 23 meaning that 22 is missing. Is there something planned for it or was it a typo?