@Override
public VoxelShape getCollisionShape(BlockState state, IBlockReader worldIn, BlockPos pos, ISelectionContext context) {
    if (context.getEntity() instanceof PlayerEntity == false) {
        System.out.print("not player!\n");
        return VoxelShapes.fullCube();
    } else {
        System.out.print("player!\n");
        return VoxelShapes.empty();
    }
}

this is my code ^

My goal is to have the block act like air for a player but act as a solid block for a mob. I know the textures aren't nice, it's just for testing right now. As you can see in the video it works fine on the y axis but not on x axis, unless I sprint. As you can see on the console at the bottom, when passing through on the x axis it thinks I'm not a player for a second? I'm completely lost. I've tried everything I can think of. Any ideas are welcome!

as demonstrated at the end for some reason there is some kind of "force" that pushes me out the block when there is not a neigbouring block. Changing everything to return VoxelShapes.empty(); works perfectly so it's not a problem that has to do with VoxelShapes.empty();

I forgot the video... 

Here's the video: https://imgur.com/a/c00hMT3

On my conditionally solid block I'm checking if the context is a dummy, but its been so long I can't remember why. (I know it was due to in issue, but that's it)

 

if (context == ISelectionContext.dummy() || context.getEntity() instanceof PlayerEntity) {
	return VoxelShapes.empty();
}
return VoxelShapes.fullCube();

 

or if you don't need the if block for other code you can make it one line.

return (context == ISelectionContext.dummy() || context.getEntity() instanceof PlayerEntity) ? VoxelShapes.empty() : VoxelShapes.fullCube();

 

you are a saint! I have been trying to fix this for quite a while already. Thanks a lot!

If i'm recalling correctly, Minecraft passes a dummy when setting up its solid block cache, and then uses that cache when checking if it needs to push an entity out of a block.

 

Glad it worked, its no fun getting stuck on something.

A few days ago I spend hours running circles on some 1.15 rendering code (not what I posted here), finding the fix was only one line was both a relief and devastating due to the wasted time.