In iClone 7.41, this problem does not occur. In the forum, when I rendered Reduce Color Banding With Noise on the rendering panel was turned on I got an answer that this phenomenon happens. I have already updated to 7.41, so 7.4 It can not be confirmed whether this switch was on.
Thank you for your feedback. Please provide project file to help us identify the problem.Attach the file in your next comment. If the file size is larger than 10MB, you can use cloud services such as Google Drive, Dropbox or OneDrive to make the files available to us and put the link in your comment. You can use Private Comment if you want to keep it private. Your help is highly appreciated. Reallusion
This has not appeared as a common issue in the forums, so I doubt it's a bug in the software that everyone has installed. (Possible, of course, but I'm doubtful.) The "random" nature of that square makes me think of "driver" issues. Please ensure your graphics card driver is up to date. If that doesn't help, please consider posting on the forum, too. Good luck.