Could be just a configuration issue; on pre-Android Pie (Android 9) phones, which should be your case, here's what you should check:
go to Settings menu, open Apps, then Notifications, make sure the ***switch*** is flipped ***on***.
Also make sure Incoming calls permission is on and importance is set to Urgent
Reference tutorial [link|https://www.smartprix.com/bytes/solved-screen-not-turning-on-with-incoming-calls/|here|new_window=true].
If this doesn't solve it, there could be something interfering with the proximity sensor's functionality. At [link|https://www.technipages.com/fix-android-phone-goes-dark-during-calls|this link|new_window=true] you'll find some useful tips give them a try, for a quick sensor test you can also go with the built-in diagnostic tool, should be accessible by simply dialing the code ''#''#0588#*#*.