Pre-installed apps (LG G2) start vanishing!!

[AC Question]

I have been an LG G2 owner for a year now and never got a problem until yesterday. My pre-installed app started to vanish one by one without leaving any traces!! First, it was my VIDEOS app which disappeared. So, to work it out I try factory resetting my phone after I had finished backing it up. After that cleaning process', I tried updating my LG backup apps and how shocked I was to see that my LG BACKUP app have vanished too. How can sth like this happened? Have u ever faced problem like this before?
I own LG G2 D802 running Lollypop 5.0.2. Kernel version 3.4.0. Phone's never been rooted before.
Need help thankss