I bought the dual rear light/brake light LED bulb, fitted it, and it worked only as rear light but not also as brake light. I took it out, turned it round, and it worked fine as dual filament.
The bulb, from SuperbrightLEDs, is a 1157-R19W6, and fits in my G5 either way round.
That's how it is. You really mustn't worry about it if you're having no problems.