That’s weird, I didn’t realize that. Might be a bug, although personally, I would consider that 2.8 is probably handling it better-- it’s weird to give different exports based on rendering settings. Blender can have no knowledge of the engine you’re going to use the .obj in, how that engine handles gamma, what kind of textures/values it expects, etc.
(I don’t like the way Blender handles gamma in a lot of ways, it tries to hide it away rather than making it part of the process, acts like it’s smart enough that users don’t have to worry about it but that’s never true. Would much rather just see sRGB->RGB and vice versa conversion nodes than all the issues that Blender creates because it thinks its smarter than the user.)
In any case, you know what to do if you want to fix it.