Cannot rule anything out... so without seeing your code, I wondered if you have a while (!Serial) in your code as this will prevent code from running until the Serial Monitor is opened.
Cannot rule anything out... so without seeing your code, I wondered if you have a while (!Serial) in your code as this will prevent code from running until the Serial Monitor is opened.
I finally got to watch the videos. Apologies. That has nothing to do with firmware. As noted by Gough Lui, the way the led fades in and out at the beginning, upon reset, is indicating that it is in bootloader mode. As such, it may be worth updating the bootloader or at least confirm that you have the latest version.
https://github.com/adafruit/Adafruit_nRF52_Bootloader
https://learn.adafruit.com/bluefruit-nrf52-feather-learning-guide/using-the-bootloader
https://learn.adafruit.com/bluefruit-nrf52-feather-learning-guide/introduction