No rear turn signal flashers while braking

[CriticalMass]

I've looked through ERAs signal troubleshooting guides but none of the circumstances given there match what I'm seeing, so I thought I'd ask for suggestions:

Brake lights work, headlights work, and all four signals work. If I turn the running lights on, and then signal, the flashing turn signals are visible because the flashers are brighter than the running lights, as they should be.

However, when I apply the brakes, which are brighter than the running lights, I can no longer see the pulsing rear signals through the strength of the brake light. If I look directly at the filament through the trunk, I can see that it is pulsing, but it's barely perceptible. From outside the car the pulsing is so weak that it just translates as a steady brake light.

The signal switch seems to be doing its job. I assume the continuity checks from ERAs troubleshooter will indicate that the switch is good, or I wouldn't see signal flashing at all (without brakes applied, either). Maybe it's still operating but in a degraded state? Anybody have ideas on how to proceed with troubleshooting?