In 1.15.2 and below, observers reading a piston base could detect that piston starting to retract. However, in 20w06a this is no longer the case.

This can easily be shown with the setup in the screenshot. In 1.15.2 and below, both observers power at the same time and the pistons pulse in sync. But in 20w06a the observer detecting the piston base triggers 2 ticks later (at the end of the piston retraction) and the pistons no longer pulse in sync.