Pistons can only be re-pulsed 5 or more gameticks later.
This bug is a direct result of MC-135194
Pistons only pulse once when given 2 pulses 4 gameticks apart. (they pulse twice in 1.12.2 and 1.13)
How to replicate:

When given 2 pulses 5 gameticks apart (see image below) pistons will pulse twice (this is intended)