I have a Pi 4 connected to a small no-name HDMI monitor. It boots and runs from an SSD. It is active 24/7, monitoring my land-line phone's incoming Caller ID signals.
Each weekend I update whatever software packages have changed recently. I don't pay particular attention to what packages update, but last time it was around 25. Also each weekend, I copy the contents of the SSD to a flash drive using SD Card Copier. This procedure has served me well for many months.
I am accustomed to the screen blanking out after a short while, but since the last update this is no longer working. I have toggled Screen Blanking under Raspberry Pi Configuration. I have fiddled (ignorantly) with xset. I have powered the Pi and its monitor down, waited a moment, powered it back up. Screen blanking is just not working anymore.
What am I missing? Before this problem, I had not changed any settings.
Once-upon-a-time I played with xscreensaver, which is no longer installed, and the entertainment value seems like overkill in this situation.