If the monitor is staying on, then it is likely that the CEC command to turn the screen off is not passing through correctly, either because the monitor doesn't support CEC properly, CEC isn't enabled for the HDMI port the monitor is connected to, or the HDMI cable used doesn't support all the required wires/signals.
In any case, the SX80 is now an obsolete unit (reaching its end of support date on 31 October 2024), so should be considered for replacement with a current and supportable model if it is going to stay in use in the longer term.
Wayne
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