@capriciousduck See link above Re: Caffeine. That seemed the only way out after some exhaustive investigation then. I recognise it just cures the symptoms, not the illness. If you, or anyone finds a proper solution, then do post it here.
Okay. I’ll check.
Done another search re same thing on Ubuntu 18.04 (same as Zorin 15). https://askubuntu.com/questions/1089769/ubuntu-18-04-will-not-stop-blanking-screen. The problem still seems persistent for some people. I do not suffer myself, so wonder if it is graphics specific.