If you have a post process material, set to After Tonemapper (and also replacing tonemapper), the gamma of the result for just that post process material is wrong. For examp if you create a post process material and just add a constant of 0.2 the viewport will show the correct grey, but when you render (either with MRQ or legacy) it’ll come out much brighter. If you add a power node of 2.2 after the constant then of course the viewport looks dark but the render will be the right value.