@Anupam_Das, unfortunately, the tonemapper is setting the limits for mapping HDR to LDR values. And in real world terms, there is no such thing as absolute white or black so this tone mapper is making some assumptions. One of those is that ACES was meant to be viewed in a dark room like a movie theater so it is balanced for that environment. If you need a different tone mapping, you will unfortunately have to create a custom tone mapping function.
If a custom tone mapper is out of the question, you could use a LUT to have complete artistic control over the final render as a post process. How To Use Post Processes | Babylon.js Documentation (babylonjs.com) and you can see an example at LUT demo | Babylon.js Playground (babylonjs.com). Hope this helps.