It seems once Matlab starts, the Ubuntu monitor won't fall into sleep, but stay on all the time on some Matlab versions.
I have searched solutions for the issue, but all I found are how to disble screen saver under Linux.
Here are my limited test:
On an Ubuntu 16.04 machine, Matlab R2017a has no problem. Even if Matlab is running, the monitor will get into sleep mode based on the setting.
On the same Ubuntu, Matlab R2019b gives the problem. Once Matlab is running, the monitor will stay on.
On another Ubuntu 18.04, both Matlab R2019b and R2020a give the same problem: monitor stays on once Matlab is running.
In all tested cases, after Matlab exists, the monitor will get into sleep mode without problem.
Any idea or workaround? Thanks.