I’m experiencing this issue with my B key and it’s annoying af. I use my MBP in clamshell mode with an external keyboard and I dread going mobile.
Mine was out of warranty, so I fixed it myself.
Failed: compressed air Failed: cleaning under the key.
Success (but not recommended) I removed the key, then pulled off the glued membrane. Cleaned all with 90% alcohol. Took a new membrane (but you might be able to use old) and used E6000 and a toothpick to glue it back. Replaced butterfly and key.
Not a good solution, but it worked.
It looked like the contacts were getting coated and not making good contact? Perhaps heat?
Best of luck.
I can crank out a site fairly quickly but I also want it to be as low-maintenance as possible.