I have no definite answer for you, but I'm having the exact same issue with my LG Optimus F3. The ONLY suggestion I've found elsewhere online is that perhaps an App I downloaded is turning Quiet Mode on. But I've been unable to rule out/verify if that is the cause. And I have very few Apps on my phone.
If it happens again tonight I am going to go ahead and actually SCHEDULE Quiet Mode to be ON from just 2-4am M-F and see if I can avoid the problem that way.
Sorry I don't have a definite answer for you.

 

I know it's been a few months, but KBeth, what was the result of you investigating the issue? Anything that might help out someone having the same problem?

CJSF

 

I have on Optimus F6 with a quiet mode issue as well. Seems to be an issue on LG's end. We recently received a firmware update, but I haven't tested it since. I gave up on quiet mode and I use trigger pro instead.

By the way my issue was a little different, I would enable quiet mode and it would disable itself upon a reboot.

 

I resolved my issue by simply scheduling the Quiet Mode to be ON from just 2-5am M-F. Even with QM set, I was also able to select numerous phone numbers(I select) to bypass Quiet Mode during that time..such as close family/friends who might need me in case of emergency during that time. No recurrence of quiet mode turning itself ON randomly since. Hope this helps!

 

Thanks, KBeth. I scheduled it for all week 7:00 to 7:01 AM and we'll see if THAT works