Water resistant means about 2psi. Even a restricted shower head produces over 15psi, so the water can get into the phone. I'm afraid that Note 9 is going to keep getting worse over time, until it just quits.
As @
kct1975 almost said,
ALL electronic devices need to be kept away from all moisture as much as possible. If you have to take a phone into the shower, put the phone into a zip lock bag, zip it shut, then put that bag into another one zip end first, and zip
it shut. And don't hold it under the falling water even then. (Just the water vapor in the shower can get into the phone if it's not protected, and when the phone cools off, the vapor condenses, and you have water inside the phone - which causes eventual corrosion.)