A common style of "Portcullis gate" uses gravity blocks like sand or gravel to push up fence posts up by at least two layers. When the pistons retract, both layers should "resettle" as solid blocks as shown in the following video:
https://youtu.be/hISevc8lq_g
On Pre6, one of the layers "breaks" so there is now only a single layer, so the fence posts can only be pushed up by one layer and the gravity block items will despawn.
It looks like something changed in the falling entity timings such that one entity falling upon another becomes "broken" faster than as happened with prior versions. This will break all "falling sand" portcullises working in prior versions, and a work-around becomes a huge problem with double-retracting pistons to pull the "gate" back down.
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Comments 3
Still broken in Pre8. Unfortunately, due to work commitments, I am not able to set up a proper test rig to verify specifics. I can verify that it is the top layer that breaks, using sand on top of the gravel layer. More details will come when I'm not working 12 days of 11 hrs a day. :-/
I believe this bug is either "connected with" or a "duplicate of" MC-134300. In any case, the problem I was having with "Mumbo's Portcullis" is working in 18w30a. Please link this bug as "relates to" MC-134300 (or flag this as a duplicate if appropriate) and set this "Resolved." If future updates seem to trigger this problem again I'll note it on MC-134300.
This bug may be related to https://bugs.mojang.com/browse/MC-132706 though I'm not clear on the details. I have confirmed this is still an issue for me in Pre7.