Copied from my forum inquiry that I eventually solved on my own...

I am a type designers & I use illustrator in building & testing fonts as part of my production workflow. I use the same array of font apps along with my cc2019 suite on 2 different machines. One is a mac mini one is an imac, both are on the latest mac os.

On the mac mini, with Illustrator open I can see changes to a font immediately reflected in the document. In other words if I replace a .otf or .ttf font file installed at mac hd/library/fonts any changes in that font immediately update in the open .ai document. If I install a new font the font will immediately pop up in the font menu.

On my imac neither of these things work. I need to quit & restart Illustrator to see new fonts, some fonts refuse to ever show up at all & I can not see any changes made to existing fonts unless I quit & restart Illustrator.

I've been searching & searching & trying everything for hours & hours & I can't fix it. Any clue would be greatly appreciated.

The solution...

Both the problem & the solution are similar to issues I was having with Dreamweaver cc2019 on the same machine. In both cases all menu based & keystroke based methods or purging &/or resetting the app preferences & cached data failed to solve the issue. In both cases I needed to find the offending pref or cache file and MANUALLY delete (or remove) it.

While it was a slightly more obscure file in the case of Dreamweaver, in the case of Illustrator the offending file was the main prefs file "Adobe Illustrator Prefs" located at:
harddrivename/users/username/Library/preferences/Adobe Illustrator 23 Settings/en_US/Adobe Illustrator Prefs

The hard reset of preferences done by option>command>shift while starting Illustrator failed to purge whatever bad data was causing Illustrator to refuse to update fonts on the fly. Only manual removal of this file fixed the problem. The file will be regenerated automatically the next time Illustrator is started up.

Purging of any of the specifically font related prefs/caches did not seem to help at all.

The app seems to behave as expected now. If I modify a font & install a new .otf or .ttf under identical names, version numbers etc. Illustrator will imediately recognize & display the changes live in the document with no need to close & reopen the doc, restart the app or the machine or reset any panels.