Basically what happens is that if you open a world with observer blocks inside of it or place observer blocks in an existing world while using Forge (doesn't seem to matter what mods are running), and you quit Minecraft and reopen your world using vanilla or Optifine, all the observers will be "frozen". Their light on their texture will be stuck to red, and they won't respond, and the only way to fix it is to either replace them or go back into Forge.
Any one else experiencing this? And why does this happen?
In order for the observers to respond again, you have to open the game in Forge 1.12. Doesn't seem to matter what mods are running.