Today I found a Dropbox tech support link that indicates that the missing overlay icons in the macOS finder “Nextcloud Extensions” could be caused by 3rd party extensions such as OneDrive. As a result, I am now testing and only allowing Dropbox and Nextcloud “Finder Extensions” to be enabled. So far the overlay icons are visible.
Please test and let me know how it works for you.
Server - 14.0.8
Desktop for Mac client 2.5.2
macOS Mojave 10.14.4