Unfortunately, I have exactly the same problem for the second time in a year.
My right headphone is suddenly quieter, I think it must be the product. The last time I was able to replace my headphones free of charge, but the effort and the phase without headphones is very annoying!

I would argue that you can also control the volume individually in the app. Under Settings --> Gestures --> Select left headphone --> Volume control. Hope that works, good luck. Otherwise hopefully warranty...