I have been exploring the topic and found a possible solution for this problem:
Open the Archviz Interior Rendering Sample Project with default settings.
Select the Post Process Volume Actor from the Outliner.
Go to the Global Illumination section (you will see that the default method of the project is Standalone Ray Traced), drop down the Ray Tracing Global Illumination group of options (you will find the Type, Max Bounces and Samples Per Pixel options), change the Type to Brute Force, this should improve the appearance of the image in the viewport.