It gets pretty humid in a fairly confined space quickly when I run the shower hot. There's enough room to setup my keyboard in there to run a humidity test. If you have similar room, you can try that to see if that breaks it. Not quite as hot as a summer day in the sunlight but plenty humid.

Same test can be done with the replacement keyboard to compare humidity susceptibility.

I'm coming to this forum 2 years late but I'm experiencing the exact same issue but with the D and A# Keys. Similar to this user it is every key of those notes regardless of octave. I don't know if anything was discovered here but I find it interesting. A factory reset doesn't seem to fix it nor does turning it off and on or anything else I've tried

Please contact an authorized service center, near you. You can find contact information in the back of your owner’s manual or through the Yamaha website.