I have a similar problem using an external HDMI monitor connected to a laptop.
It happens with the blank screen after a period of inactivity (without using the screen lock).
After the screen has gone blank due to a period of inactivity, I move the mouse or press a key on the keyboard to wake up both screens. However, 1 of 3 things will happen:
1. The laptop's screen will wake up but the external monitor doesn't wake up.
2. The external monitor will wake up but the laptop's screen doesn't wake up.
3. Both screens will wake up as they should.
To get both screens to wake up, I right click on the active screen and click "Display Settings". Click on "Mirror" then back to "Join", then "Apply" then "Revert Settings". The blank screen then realizes it is supposed to be on and it wakes up. The other option is to unplug and re-plug the HDMI cable in, but I got tired of doing this.
I have a similar problem using an external HDMI monitor connected to a laptop.
It happens with the blank screen after a period of inactivity (without using the screen lock).
After the screen has gone blank due to a period of inactivity, I move the mouse or press a key on the keyboard to wake up both screens. However, 1 of 3 things will happen:
1. The laptop's screen will wake up but the external monitor doesn't wake up.
2. The external monitor will wake up but the laptop's screen doesn't wake up.
3. Both screens will wake up as they should.
To get both screens to wake up, I right click on the active screen and click "Display Settings". Click on "Mirror" then back to "Join", then "Apply" then "Revert Settings". The blank screen then realizes it is supposed to be on and it wakes up. The other option is to unplug and re-plug the HDMI cable in, but I got tired of doing this.
OS: Ubuntu 22.10
Graphics: Mesa Intel® HD Graphics 5500 (BDW GT2)
Windowing System: Wayland
Processor: Intel® Core™ i7-5500U × 4