I have a a5 2017 and been having this problem nothing has worked for me. I have found a solution. If you go settings then apps, select the 3 dots in top right and select app permissions. Scroll down and select telephone access. you will find all the apps what are connected to telephone access. I disabled the ones I didn't think I needed which originally I had 25 selected and one of these was creating conflict but didn't know which one it was. I got it down to 4 selected and now I have my missed call notifications back. I don't know what app was causing this but I'm glad it's sorted hope this helps someone as the original soloutions didn't work for me and I didn't even have true caller installed.
here's the solution
Make sure you’ve enabled to show icon badges:
1. Pinch a home screen using two fingers.
2. Tap Home screen settings > App icon badges.
3. Turn on this feature and select Show with number for the Badge style.
It has always been on. I receive notifications on all of my apps except missed calls. I see a notification when i get a voicemail as well, but never on missed calls