There is only one lights output, so if you choose flashing, it flashes both front and rear.
If the rear light does not flash, it is because you have used a dynamo light, which has a reserve of power to stay on for a few minutes when the bike is stationary.
If the rear light does not flash, it is because you have used a dynamo light, which has a reserve of power to stay on for a few minutes when the bike is stationary.