My friend I installed Android 9(old version) a month ago after I started facing screen blackout/shutdown issue. I've used Android 9 for more than a month and I still have the issue. I can guarantee you they are fooling you. Installing old version of Android will not fix the issue. Go to service center ,complain that issue is not fixed and ask for replacement since you're still in warranty. Don't settle for their fake promises. Don't settle till you get replacement.
Edit//:Thanks for the update
Thanks for the advice will surely do that. By the way have you been able to get the replacement for your phone?
No I am yet to visit the service center because of lockdown and sudden increase of cases in my area. Having this issue for nearly 2 months now. Disappointed.