I've tried on both my own custom screen and the included forge_duplicate and in both cases the image loaded in the background flickers while the game loads.

Minecraft 1.12.2
Forge 14.23.5.2847
CustomLoadingScreen 1.4.1

I haven't worked on custom loading screen (properly) for quite a while, so it might be quite a while until I take a look at this :/

(While this doesn't happen to me I've only tested with 14.23.5.2768, so something might have changed in forge itself since then)