Likely you have a case where the brake light filaments in the 1157 bulb have a missing/bad ground connection a the socket. In this case the filaments can find ground through the parking/running light filaments when they are off. When those lights are on, you then have 12 volts at both sides of the light and no current flows. In that case you would also see that the rear running lights will turn off when you apply the brakes.

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