it looks like an issue with using uhd displays. here is a link that explains the situations.
https://www.dell.com/support/kbdoc/en-us/000125427/issues-when-using-uhd-4k-x-2k-and-uhd-5k-x-3k-displays
Now where you may have to make this adjustment on your mac is an issue. Check your display settings and see if there is a setting similar to what is mentioned in the article.
The other place to look will be in RT itself and check how it handle uhd displays.