Hi Daniel, I have figured out the cause of the problem. I had previously installed a program called “RedQuits” that terminates any program—as if by typing Command-Q—when the red “x” button on the top-left of any application window is pressed. (This was to emulate the WindowsOS functionality—I installed it when I first switched from Windows to Mac.)
Since the VST plug-in window has those three window buttons, I can only assume the RedQuits program activated once the “x” button was pressed and proceeded to terminate the audio engine. Sure enough, once I had deactivated the RedQuits program, the problem ceased.
I feel a bit silly now for not having made this correlation until it was pointed out to me. Thank you all for your help, and I sincerely apologize to both you and Ulf for the trouble I have caused.
Hopefully, this thread will serve as a warning (albeit niche) to consider if a third-party program might be the cause of a performance issue.