This is a niggle more than anything, but after reporting to Adobe Support they were more than useless about providing a satisfactory answer or fixing it.
Can I just put this out there for anyone running Illustrator CC2019 on Mac OS Mojave 10.14.1?
If you right click and bring up the contextual "open with" menu, does your (default) application show as Preview or Illustrator CC 2019? Mine shows Preview.
Now, this doesn't matter if you go and select Illustrator CC 2019 to open all files etc and change it. Preview refuses to budge and still shows in the "Open with" menu.
The reason I said this is a niggle is that it doesn't affect the opening of .ai files or other native Illustrator files and they will open in their chosen default of Illustrator CC 2019 application automatically. It is just this damned "Open with" menu that bugs my OCD. 
There was originally mention of aigpusniffer being a culprit, but I don't see it affecting any other app. Either way there is obviously a mini bug there and Adobe refuses to accept responsibility here.
I would be interested if other users see the same issue that I do, so i know i am not going crazy!
Thanks
AdChoices