The picture is a better explanation than I could make, I'm not sure what's the cause. Effects this and Portalgun.
It also causes the Terminal and second portal (it's happened to both but I'm not sure why it chooses whichever) to disappear.

Do you have optifine installed? For me it have worked, but with OptiFine I had the save issue.

hey i managed to fix this issue while messing with settings, what you need to do is turn fog off completely and that should fix the issue

hey i managed to fix this issue while messing with settings, what you need to do is turn fog off completely and that should fix the issue

You, my dude. I hope you have the best fucking day in your entire life.

:OO