Mobs no longer see buttons as full blocks in Bedrock Beta 1.16.100.55. This is likely due to improvements to mob pathfinding. Here's a video I made demonstrating the bug: https://www.youtube.com/watch?v=3Ek140awEaA
Existing mob farm designs rely on buttons to improve the efficiency of drop shafts, and this no longer works in the Beta. This a behavior that has existed in the game for years.
Player Expectation: Mobs see a button placed on the side of a block as a full block and pathfind over it, falling into a gap.
Game Behavior: Mobs avoid pathfinding onto buttons placed on the side of a block.
Consequences: Players spend days or weeks building multi-layered mob farms in their worlds and depend on the button mechanic to make these farms work. In the beta, these farms no longer work and players will not know why.
Suggested Change: Mobs should treat buttons placed on the side of a block as full blocks when pathfinding.
Precedence: The impact of this bug is similar to previous changes to slime chunk locations and pressure plates that were reverted because they broke existing farms.
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Reopened 🙂

Thanks, Urielsalis.
I really feel like this is similar to the recent changes to collision boxes for pressure plates that broke certain farms and were reverted. MCPE-80276
I think it is important to discuss if any of the improvements to pathfinding have unintended consequences that should be addressed or not. This is a mechanic that has been in the game for a very, very long time and well established mob farm designs are impacted by it. It is smart for the developers to consider the impact before finalizing the change. If needed, we can post lots of tutorials and let's plays from YouTube that use this mechanic.

Also, I want to make clear that I am specifically talking about buttons here. The other blocks that were updated in connection with buttons are not used in pathfinding-based farms as far as I know.

You can resolve this ticket and the related button pathfinding issues by let mobs only pathfind across buttons placed on the sides of blocks. This will not affect how mobs interact with buttons place on the top or bottom of blocks.

There is precedence for what Reed suggests in the comment just above, in the fix for MCPE-81316. That issue was fixed by changing the effect of collisionless blocks on a boat when placed on top of the ice the boat travels on, without changing the effect of those blocks on a boat when they are placed on the side of the ice that a boat travels on. So now blocks like buttons and torches are handled differently when in the same block as a boat than they are when in the block below. Doing the same for buttons and pathfinding could fix the issue reported here while preserving the fixes to MCPE-47075 and MCPE-44575.
See
[media]for what I mean.

Happening again. All of my mob farms that use buttons on the side of blocks so mobs will pathfind across them stopped working after the latest hotfix.
PS4

This is still an issue for me in 1.16.200 on Windows 10 edition. I used this design: https://www.youtube.com/watch?v=9G2hpEVuXXM

Is this going to be fixed or do I need to rebuild my creeper farm that I spent weeks building (from build height to bedrock)?

I think this is a parity change. Don't open trapdoors work the same?

Affects 1.17.40.20

This is a bug not a feature just use coral fans they reverted the change they made that made mobs don't treat coral fans as full blocks.

This was fixed with the fix for MCPE-153897.
Please don't mark it "Resolved" so quickly. Jay requested an original bug report on the breaking of Mob Farms: https://twitter.com/Mega_Spud/status/1303087436004823046