[BUG] [1.12.2] Vanilla flying machines duplicating blocks....sorta

[BagginsBlocks]

That's not supposed to happen...

Latest version of forge, no additional mods installed. Duped blocks can be interacted with to a degree. You can stand on them, and break them, and push them with pistons, but if you try to place a block on one the glitched block is replaced by the one you're placing. 

From what I can tell, this same bug has appeared in snapshot versions of the vanilla game before. No Idea how forge reintroduced it, but it doesn't occur if I run the game vanilla. 

My "latest" log, so I won't be banned:

Spoiler

[23:43:57] [main/INFO]: Setting user: Bagladon
[23:44:01] [main/WARN]: Skipping bad option: lastServer:
[23:44:01] [main/INFO]: LWJGL Version: 2.9.4
[23:44:05] [main/INFO]: Reloading ResourceManager: Default, FMLFileResourcePack:Forge Mod Loader, FMLFileResourcePack:Minecraft Forge, FMLFileResourcePack:mercurius_updater, f81517ca516363c381528480bb4c5189-New Default Betapack V2 (1).zip
[23:44:06] [Sound Library Loader/INFO]: Starting up SoundSystem...
[23:44:06] [Thread-6/INFO]: Initializing LWJGL OpenAL
[23:44:06] [Thread-6/INFO]: (The LWJGL binding of OpenAL.  For more information, see http://www.lwjgl.org)
[23:44:06] [Thread-6/INFO]: OpenAL initialized.
[23:44:06] [Sound Library Loader/INFO]: Sound engine started
[23:44:10] [main/INFO]: Created: 512x512 textures-atlas
[23:44:12] [main/WARN]: Skipping bad option: lastServer:
[23:44:12] [main/INFO]: Narrator library for x64 successfully loaded
[23:44:19] [Server thread/INFO]: Starting integrated minecraft server version 1.12.2
[23:44:19] [Server thread/INFO]: Generating keypair
[23:44:20] [Server thread/INFO]: Loaded 488 advancements
[23:44:20] [Server thread/INFO]: Preparing start region for level 0
[23:44:21] [Server thread/INFO]: Changing view distance to 19, from 10
[23:44:23] [Server thread/INFO]: Bagladon[local:E:d7853dff] logged in with entity id 10 at (14.91525173259367, 69.82842457581482, 339.0197610090509)
[23:44:23] [Server thread/INFO]: Bagladon joined the game
[23:44:24] [Server thread/INFO]: Saving and pausing game...
[23:44:24] [Server thread/INFO]: Saving chunks for level 'Redstoned'/overworld
[23:44:24] [Server thread/INFO]: Saving chunks for level 'Redstoned'/the_nether
[23:44:24] [Server thread/INFO]: Saving chunks for level 'Redstoned'/the_end
[23:44:24] [main/INFO]: Loaded 460 advancements
[23:46:13] [main/INFO]: [CHAT] Saved screenshot as 2018-02-28_23.46.13.png
[23:46:16] [main/INFO]: [CHAT] Saved screenshot as 2018-02-28_23.46.16.png
[23:46:57] [main/INFO]: Stopping!
[23:46:57] [Server thread/INFO]: Stopping server
[23:46:57] [Server thread/INFO]: Saving players
[23:46:57] [Server thread/INFO]: Saving worlds
[23:46:57] [Server thread/INFO]: Saving chunks for level 'Redstoned'/overworld
[23:46:57] [Server thread/INFO]: Saving chunks for level 'Redstoned'/the_nether
[23:46:57] [Server thread/INFO]: Saving chunks for level 'Redstoned'/the_end
[23:46:57] [main/INFO]: SoundSystem shutting down...
[23:46:57] [main/WARN]: Author: Paul Lamb, www.paulscode.com
 

Please help? This bug breaks the very reason I was running a modded client to begin with, and invalidates some of my recent life choices.