Thank you for taking the time to present your issue in simple & readable manner.
Well, it looks like MARI works this way:
So what you observe, is correct behavior & with that in mind, results are consistent all over, in other apps and engines (although a few adapt shaders automatically ie. Unity:
What the shader does for the albedo and specular is "linearize" the maps. It removes the gamma-encoded values from the map in the shader code by applying an inverse gamma to it of 0.4545. In the Unity workflow, this is done automatically and you don't need to flag the images as sRGB.
My conclusion is - It's nothing wrong with Corona (except the need to do this automatically) -
feature request! ;)
eg. CentiLeo settings for comparison: