I have a VGA monitor connected with a DP to VGA cable.
I have disabled the system’s built-in display power manager so it does not interfere with tests.
Then I run this command: xset dpms 30 60 90
Result: After 30 seconds my monitor switches off. At 60 seconds it turns back on. At 90 seconds it turns off again.
It does not matter which DPMS modes are enabled or in which order. The first one turns off the monitor, the second one make it turn on again, and the third, if enabled, will turn it off.
It does not seem to happen if the delay between modes is very short eg xset dpms 5 10 15 will not reproduce it.
The problem is related to the DP to VGA cable. The problem is reproducible with different monitors and the same cable. With a different cable, those same monitors do not reproduce it.