Summary
Two observers facing into each other form a clock, as intended. However, in 1.13 the resulting clock has the wrong timings.
Expected behavior
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.
The bug
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.
Why this should be fixed
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.
Related issues
is duplicated by
Attachments
Comments


Another reason to fix: This change implies something has changed with how observers react to change.
The reason why pushing with a piston created a 4gt clock is that when the piston pushes, it places the piston extension (aka block36) directly in front of the observer, which the observer detects. This creates the offset of 2gt between the observer pair, making them trigger every 4gt by detecting each other's change.

Can confirm for 18w20a.
This is especially troubling as it breaks a great deal of redstone contraptions.
Here is setup testing timing outputs with command blocks:
[media]
present in 18w20b

present in 18w20c

present in 18w21a

present in 18w21b

present in 18w22a

present in 18w22b

present in 18w22c

Affects 1.13-pre1...

present in 1.13-pre2

1.13pre3 affected.

This seems to be fixed in pre4, can someone confirm?

Can confirm that it at least appears to be fixed.

Affects 1.14.1 pre2