I'm not entirely sure how to explain what is going on here, but what I do know is that this bug breaks many previous redstone contraptions.

Despite each one of these 14 different circuits having the same 2 game ticks of delay between the top and bottom observers, the top piston is only able to retract the bottom piston in 3 of the circuits in 1.13. All 14 of these circuits retract properly in 1.12.

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