Strange bug in note drum note input

I’m working through John Barron’s excellent tutorial on drum notation (https://www.youtube.com/watch?v=Y31VOP26TJY) and have hit a snag when working through the first example.

When entering triplets as indicated in the video, using the MIDI keyboard to input notes yields this when getting to beat 2. What am I missing?
76631267_10158250146864903_320237679327313920_n.jpg
75231829_10158250147659903_5062171454339022848_n.jpg