The following Apple Technical Service Bulletin has been released by Apple:

Apple Technical Service Bulletin

This document provides additional instructions for operating the Mac Mini (Late 2018) macOS Catalina 10.15

Mac Mini (Late 2018) users should refrain from upgrading to macOS Catalina 10.15. macOS Catalina - Apple
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Published Date: October 12, 2019

Apparently, the installation of Catalina updates the firmware of the Mini which in turn renders external monitors to no longer display the proper resolutions. Folks have reported that even reverting back to Mojave does not cure the problem because of the new firmware which stays with the Mini.

If you have not installed Catalina and are thinking about it and own a 2018 Mini, hold off until Apple provides a fix.