This bug is a direct result of MC-135194

As the name entails, pistons can't be powered by a 1 tick pulse 2 gameticks after being moved.

This glitch is rather conditional in 18w30b because there are many, many affected circuits that despite having the same delays behave differently. To show this, I have created two videos; one for the behavior in 1.12.2/1.13/18w30a (all have versions same behaviors) and one for 18w30b.

1.12.2/1.13/18w30a behavior: https://www.youtube.com/watch?v=oa3MIC9t7ps&feature=youtu.be

18w30b behavior: https://www.youtube.com/watch?v=TRXCFwFIylc&feature=youtu.be

I believe this is a problem with pistons and not observers because the circuit below contains no observers and exhibits similar behavior.

In 18w30b, the front piston ejects the block forwards as if being 1 ticked.
In other versions, the front piston extends and then retracts the block.

 STRUCTURE FILE IN VIDEOS ATTACHED BELOW