This might be a long shot, but in Linux with had a problem with GL_FRAMEBUFFER_SRGB which relates to sRGB conversion, where we had to disable to get it to look normal and not washed-off.
This problem appears to be the opposite, being darker so perhaps it is not converting to sRGB space…so if you don’t mind some hacking with the code, look at Engine/Source/Runtime/OpenGLDrv/Private/Mac/OpenGLMac.cpp for some clues.