I've encountered almost exactly this query on either this or the TSSC forum, not that long ago. It's different to the indicator question we talked about at the meeting.
There is no intentional "deviousness" in the stop/tail light circuits. The intention is that the bulb is always earthed and the circuits are thus independent. If you have an earth fault (which is very common with those light units) on one side only, the tail lights will still appear to work (if a bit dim) if they are filament ones, because the current can flow through the brake light filament of that bulb to the brake light filament of the other side and thus to earth. However, the faulty side will just "go out" when the brakes are applied. This won't work for LEDs but is, in any case, a fault condition.
Unfortunately I don't remember the conclusion of the previous discussion of LED stop/tail lights but I have a suspicion we concluded the "bulbs" were duff, possibly by design.
Try this: on a bench, with a battery, connect the bulb barrel to negative. Take a wire from positive to each contact in turn. One should light it dimly, the other should light it bright. Then connect the positive wire to both contacts together. It should light brightly. Any deviation from that behaviour is a duff LED bulb.