Observers are missing their cooldown.
Observer can power directly after the tick it already power, sending a constant power if made to update every tick

This is a huge flaw and it breaks about 80% of doors

Example: 

1.13pre-7: piston is powered for 2 ticks (4 gameticks)
1.12: piston is powered for 1 ticks (2 gameticks)

Example 2: 
1.13pre-7 : right piston is extended same way as left piston
1.12: right piston pulses twice

Example 3:

part 1: Piston
1.13pre-7: Piston is constantly powered
1.12: Piston pulses

part 2: Dust behavior.
1.13pre-7: the redstone dust never unpowers 
1.12: the redstone dust turns on and off 

+ observers powers more times/minute in 1.13pre-7 compared to 1.12