Maybe it’ll be useful to someone googling this but I had the exact same problem and I fixed it the same way we fix drifting joy-cons : I blasted contact cleaner inside the pad by opening the right cover. Put a good amount of it inside and let it dry for a minute or two. It worked perfectly and I’m now able to use a pad I didn’t use for two years (I used to play to Dark Souls 3 with it, the right bumper is essential to this game).
I used WD-40 contact cleaner by the way.