I have got the same problem and I have tried everything you could think of and it didn't work. I reinstalled the app and I did a factory setting and erased everything on my phone and the problem remained unsolved. I am getting really frustrated.
Hey guys, I'm using Nokia 6, Android 8.1.0. Was having the WhatsApp custom notification problem with the latest WhatsApp version 2.18.97, then after reading above suggestions about beta, I joined the beta programme via web browser. I can now use custom notifications (although only the call notification refuses to change from default setting!), and the WhatsApp version is now 2.18.100. Hope this helps.