This happens when I am talking on my phone also. It is very annoying and when I am leaving a voicemail, I can't get to the end call button to end the call without unlocking my phone. This problem is enough to make me go back to a iPhone!
Just got my S9 and using it for conference calls where I am on speaker. Naturally I mute myself until I need to talk. While pressing the on button brings the screen back, it's annoying and shoudl NOT be necessary.
I really am disappointed though that on a Samsung commmunity, no one bothers to address this. Obviously, there is no answer other than "it's a feature not a bug" - but come on - at least have the decency to admiot that.
AND if I'm texting ,, it goes dark b4 I can answer. AND..I use it mostly for recipes. I'm tired of having to clean my stick fingerprints off of the screen and Off button!!
This is my first Samsung...and it will probably be my last.
I've read through the whole thread and Samsung has still not presented a solution for this incredibly ANNOYING lack of phone functionality. A black screen is a pain in the !
Keep Screen On is a fantastic app. Free, no ads, no trail periods, no restrictions, no in-app purchases. I have tried this on Samsung S9 and it works for keeping screen on during speakerphone call as well. You can use this app to keep the screen on during other apps as well.
Once you open the app, click on Setting button at the right bottom, click on "Show System Apps", then scroll down all the way to "System Apps". Find apps named "Phone" (there will be 3) and click on all 3.
Use the app "Keep Screen On" from the play store.
Solves the problem.