The bug
A piston placed at Y=1 will not push downwards. This also happens at Y=254 with an upward facing piston.
Code analysis
Based on 1.11 decompiled using MCP 9.35 rc1
The method net.minecraft.block.BlockPistonBase.canPush(IBlockState, World, BlockPos, EnumFacing, boolean) allows piston movement only if the piston is not facing downwards or the potential block to move is not at y 0. It should allow piston movement as well if the block is air or has EnumPushReaction.DESTROY as push reaction.
Linked issues
is duplicated by 1
Comments 13
No, it could still do that if you just put it at Y=2 and have it push 2 blocks in a row. If you do that, it just won't extend, and that's what should happen at Y=1.
Is this still an issue in the latest snapshot 16w44a? If so please update the affected versions.
This is an automated comment on any open or reopened issue with out-of-date affected versions.
WAI. This is so pistons don't push blocks into the void or the height limit (which will disappear).