Slime block piston arrangement duplicates sticky piston

I was attempting to make a slime block gate based on https://youtu.be/F7JkZvsG5WM and in the middle of the video there is a test showing just the redstone portion. I used cobble for parts not touching the gate.

In the following three images, switching to lower extends all but the last piston, though the repeater delays are set correctly. Toggling back off doesn't retract properly, toggling it on again creates a duplicate sticky piston. When I initially tried this, I missed some cobble stuck to the piston, and it appeared to duplicate the cobble as well. However, I removed the cobble to see if that was a factor, and the screenshots show what I can duplicate.