Oh, disabling the splash screen actually does fix this problem. I originally only found minecraft/config/splash.properties and changing it there did not help, but I now see I have a minecraft/1.12/config/splash.properties (1.12 is the game data directory I created for all 1.12 versions) and changing "enabled=true" to enabled=false" there fixed fullscreen graphics.
I have the same problem. I can reproduce this on both the oldest and newest versions of Forge (1.12.2-22.214.171.1246 and 1.12.2-126.96.36.19968). Disabling the splash screen (suggested for the same problem on 1.7) did not fix it. However, I was able to work around the problem by, instead of having minecraft switch to fullscreen mode, click the Mac standard green circle in the upper left corner of the minecraft window. This put minecraft into full-screen mode with working graphics. I can then exit fullscreen by pressing the key I assigned to toggle fullscreen mode twice (the first stays in fullscreen but reduces the graphics to 1/4 of the screen, the 2nd restores non-fullscreen mode with working graphics). So the work-around is mildly annoying but working well enough for me. Hope it helps you. Tye