Good News! 😃
Apparently this is a widespread bug on macOS High Sierra that was fixed in version 6.3.10 that was released Nov. 15th 2017.
They fixed it as described in the release notes:
https://dev.mysql.com/doc/relnotes/workbench/en/wb-news-6-3-10.html
Executing a query in MySQL Workbench on a host running macOS High
Sierra failed to load the result grid completely, which prevented
result data from appearing within the Result Grid tab. (Bug #26826418,
Bug #87714)
Download it here:
https://dev.mysql.com/downloads/workbench/
(No need to uninstall the previous version, just overwrite it with the newer one)