Minecraft 1.13-pre6, Minecraft 1.13-pre7, Minecraft 1.13-pre8, Minecraft 1.13-pre10, Minecraft 1.13, Minecraft 18w30a
Up until 1.13 pre4, observers which detect a 4 gametick signal (equivalent to 2 ticks on a repeater) output a 2 gametick signal, meaning a sticky piston activated by the output signal would drop its block.
In pre 4, this behavior got changed. Observers which detect a 4 gametick signal now output a 4 gametick signal, meaning a piston wouldn't drop its block anymore.
Steps to Reproduce:
1. Build the contraption shown in the picture
2. Activate the left observer
3. Notice how the piston doesn`t drop its block, but retracts it again
This bug is probably the reason for
MC-132762 and MC-132143. Although consequences of this bug are named in those bug reports, they don't mention the reason why those problems happen. That's why I created a new bug report, although you might consider it as a duplicate of one of the other bug reports.