I’m sorry I’m late to the game on this one.
Shannon said more clearly what I would have said. But re-reading, I remember that the Atlas sensors are special. You can’t cut the power to the Atlas circuits while they’re connected to the I2C bus of the Mayfly or your program will crash. The Atlas circuits have the I2C lines pulled up to the voltage in line. If you set the voltage in to ground (ie, turn it off) the I2C lines get pulled down as well. The Arduino I2C (Wire) library hangs forever when that happens. So if you want to power down your Atlas sensors you need to get an isolator.