Two observers facing into each other form a clock, as intended. However, in 1.13 the resulting clock has the wrong timings.
In 1.11 and 1.12, pushing/pulling an observer (using a piston) in front of another observer creates a 4 gametick clock. The observers are out of phase: one is on for 2 gameticks while the other off for 2 gameticks, and then they switch.
In the 1.13 snapshots, the same clock has a 6 gametick cycle. Each observer is on for 2 gameticks then off for 4 gameticks.
This behavior is wildly used to create fast clocks. A 4 gametick clock is useful as it matches the speed of droppers. By contrast, 6 gametick clocks have very few practical uses.