.kicad_sym file is for 5.99, not for 5.1. Have you tried some development version on your machine at some point? You can try to rename the configuration folder and start from scratch, Where are the configuration files (settings, library tables)?.
No, I haven’t tried any development versions. I’ve first installed the ver. 5.1.9 yesterday from the stable page. But I had to clone the kicad-footprints and the kicad-symbols and select the global symbols/footprints files from there. I’m not sure if that could be the RC.
If you cloned from gitlab you’ve got the libraries for version 5.99/6. The official Mac package should have the correct libraries. I think there was some 5.1 testing version package at some point which had the wrong libraries bundled with it. The latest one should be fixed. The 5.1.9 final release version should be fine, too, and it’s the latest official release.
Remove the old configuration and re-install the latest package.