Mitsubishi calls it a buzzer. Set TBBZR or BZR parameter to 0
Check $LOGBOOK variable group, inside there are specific items that can be enabled/disabled.
In your situation should be $LOG_PRGEXE 0, I think?
Check "Software Reference Manual" or could guess some by the names.
There are variables to set how much space should be left in controller memory after logbook buffer(otherwise buffer is deleted).
$DRAM_MARGIN, $SRAM_MARGIN - kilobytes. But didn't have any need to change it, filtering works fine.
I don't think it resets after power off.
Also forgot that alarm history can show you time.
Click on F5 (Detail) on selected alarm.
There is an alarm "SRVO-012 Power failure recovery", it comes up after hot start.
You could check MENU>ALARM>F3 (HIST) or Motion Log. But keep in mind that this alarm can be out flooded with other alarms during production.
Or you could check LOGBOOK.LS in MD: directory (or make AOA and open with notepad). It would include robot time as well like this.
Power failure recovery
25-MAR-23 19:58 :36
It again can be out flooded by other stuff (for example "ENTER is pressed"), but those can be disabled in $LOGBOOK variable group. There is are a lot, but some could be guest like $LOG_UF means UserFrame. Or check the "Software Reference Manual".