Hello @COCKTALIS,
Sorry to hear about this experience. Would you mind removing and reinstalling Illustrator to check if it helps?
Note: Kindly choose the option to Keep preferences while removing Illustrator to retain your custom brushes, workspace, and other settings.
If the problem persists, kindly try the suggestions shared in this help article (https://adobe.ly/46Fhxn7) to change the default application and share your observations.
Looking forward to hearing from you.
Thanks,
Anubhav