My slime block sweeper nether mob farm was working normally up through 1.13-pre3, but with pre-4 it seems the timing of the redstone signal has changed, causing the traveling pistons to no longer sweep back and forth. Instead, they just jitter in place. Normally a clock sends a single tick pulse which becomes two quick pulses in succession, driving the piston pair to travel along a rail. Now the pistons act as if it is one single longer pulse and just stay in place.
It uses a clock with two observers and two repeaters - one on 1 tick, one on 2 ticks to drive the signal.
EDIT: It definitely has to do with the observer clock. I made some changes, and now the sweepers work. I attached two pictures of the changes I had to make.
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Thanks for the explanation, you worded it a lot better than I did. I wasn't sure what the problem really was, just that it was a change in observer mechanics. That makes a lot of sense to me.
The reason your problem happens is that the behavior of observers changed in prerelease 4. 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.
This is a fundamental change in the behavior of observers and would cause many redstone contraptions to break in 1.13 if this bug doesn't get fixed before the official 1.13 release.
Question to the mods: Should I create a new bug report which describes the issue in a better way? I'm asking because this bug report doesn't describe the reason why the redstone contraption broke and it would maybe be a good idea to create a new bug report which is only about the change in the behavior of observers.