How to really disable the screen when the phone is on my ear? I have a Sony Xperia with Android 8.0.0 Oreo.

When I speak on the phone, I first turn off the screen with the side button, then put the phone on my ear.
When the phone is on my ear, the screen often turn on and my fingers or my cheek or whatever touch the screen and do not-so-funny things.

How can I force the screen to remain off, or the touch-screen off until I use the side button again to turn it on?

edit: no "double tap to wake" option on this phone
It seems to be an issue with proximity sensor malfunctioning. Instead of forcing the screen to be off, you can try :


There are many apps on Play Store, search for "Proximity Sensor". Proximity Service seems to be a good choice. It has a good rating, has been updated last month and doesn't have any trackers (no privacy issues). If this doesn't work, try other apps. 
You may want to go the automation route as explained here Screen turns off during call due to problem with proximity sensor - Xperia Z3 compact (case there is opposite of yours, so it needs to be tweaked) 
Per this blog (though for a different model of Xperia) , you can test and reset proximity sensor by dialing *#*#7378423#*#*