Thanks “geeksville”. What you are reporting certainly seems related to my problem. However in the firmware for the Feather nRF52840, Adafruit_TinyUSB_Core_init() - where the hang occurs - is called before vTaskStartScheduler(). I never get to stage of being able to not step over vTaskStartScheduler.
The hang occurs in Adafruit_TinyUSB_Core_init whether I step over it or not. It seems that the rtos is using an interrupt that conflicts with the debugger.