[MC-155758] Endermen don't pick up non collidable blocks

The bug

Endermen are not able to pick up any block which is not collidable, for instance flowers or mushrooms, even if those blocks have the enderman_holdable tag.

How to reproduce

Place mushrooms in a stone covered area
Spawn a lot of endermen
Notice endermen are not able to pick up the mushrooms blocks.

Explanation

The enderman_holdable tag is added to mushrooms and flowers. Before 1.13, the decompiled source code showed that they had to be able to pick up those blocks and they didn't either. And as I made a data pack that lets them only pick up tall grass, lilypads, and other non-collidable blocks, they just pick up nothing...

I assigned a data pack I made to test the expected behaviour. The following table shows the current behaviour.

Block in data packs	Grab?
Soul sand	Yes
Any flower pot, with or without plant	No
Regular torch	No
Any rails	No
Any slab	No
Any button	No
Any carpet	No
Any door	No
End rods	Yes
The blocks below are in the vanilla data pack
Grass blocks	Yes
Dirt	Yes
Coarse dirt	Yes
Podzol	Yes
Sand / red sand	Yes
Gravel	Yes
Dandelion	No
Poppy	No
Blue orchid	No
Allium	No
Azure Bluet	No
Red tulip	No
Orange tulip	No
White tulip	No
Pink tulip	No
Oxeye daisy	No
Any mushroom	No
TNT	Yes
Cactus	Yes
Clay	Yes
Pumpkin	Yes
Carved pumkin	Yes
Melon	Yes
Mycelium	Yes
Netherrack	Yes

Eventually, it seems that endermen only pick up collidable blocks of which their shape contains the block-local coordinates [0.5, 0.5, 0.5].

Code analysis

In the MCP remapped source code, in the take-block AI, there is this:

      public void tick() {
         Random random = this.enderman.getRNG();
         World world = this.enderman.world;
         int i = MathHelper.floor(this.enderman.posX - 2.0D + random.nextDouble() * 4.0D);
         int j = MathHelper.floor(this.enderman.posY + random.nextDouble() * 3.0D);
         int k = MathHelper.floor(this.enderman.posZ - 2.0D + random.nextDouble() * 4.0D);
         BlockPos blockpos = new BlockPos(i, j, k);
         IBlockState iblockstate = world.getBlockState(blockpos);
         Block block = iblockstate.getBlock();
         RayTraceResult raytraceresult = world.rayTraceBlocks(new Vec3d((double)((float)MathHelper.floor(this.enderman.posX) + 0.5F), (double)((float)j + 0.5F), (double)((float)MathHelper.floor(this.enderman.posZ) + 0.5F)), new Vec3d((double)((float)i + 0.5F), (double)((float)j + 0.5F), (double)((float)k + 0.5F)), RayTraceFluidMode.NEVER, true, false);
         boolean flag = raytraceresult != null && raytraceresult.getBlockPos().equals(blockpos); // It's this rule that causes the bug
         // It should be: raytraceresult == null || raytraceresult.getBlockPos().equals(blockpos);
         if (block.isIn(BlockTags.ENDERMAN_HOLDABLE) && flag) {
            this.enderman.func_195406_b(iblockstate);
            world.removeBlock(blockpos);
         }
      }

Original information

As far as I know, endermen are able to pick up mushrooms, flowers and other non-solid blocks. I've seen them doing that in bedrock edition, but never in java edition. This is a problem that's confusing me for two years now, and it seems that it never showed up here...

Since endermen could pick up mushrooms, they should start grabbing the mushrooms. In bedrock/pocket edition, this is the case. But in java edition, they only grab the collidable blocks...