Why does Maya do this - and what is the solution?
The problem is a gamma correction / Linear workflow issue. Maya's Frame Buffer applies a de-gamma (gamma correction) to the output images, but the settings being used by the Batch Renderer are doing the opposite - and may be applying incorrect gamma settings to images output as part of the batch.
There are various possible solutions to the problem. One simple solution is to ensure that, when you go to your Render Settings, you tick the box labelled "apply Output Transform to Renderer". If you are lighting your shot with Physical Sun and Sky, this should solve the problem.
Another possible solution is to try saving the images as a different format. Instead of .png try using .exr instead. The reason sometimes EXR works is that it is a HDR 32bit floating point file - not a 8 bit LDR file (like a JPEG, for example). If this is the reason for the batch render being darker it would mean the brightness values above 100% are being clamped by the (non EXR) 8 bit file format which would make a file appear a little darker and duller.
The problem is also fixable by adjusting the exposure of your images in Premiere or After Effects. The preview render window in Maya adds its own post processing but when you export you get a raw image. However all the colour data is still there, and can be manipulated in editing software.
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