Hey there, want to hear something strange? According to Software updates: LG K20 Plus , there isn't an F update at all. I wonder if the page just hasn't been updated yet?
Anywhoozle, so sorry to hear of your troubles, I know it has to be super annoying to have to charge your phone all the time! Have you gone over the steps at Battery life: LG K20 Plus ? Also, some folks report an improvement by resetting a phone after a major update Device resets: LG K20 Plus . I'm not sure if the same would apply to a security update but it's certainly worth a try what with the problems you're having. It definitely wouldn't hurt! Do you have your contacts and photos backed up? Backup & restore: LG K20 Plus
you'll need to clear the cache for all of the apps on the phone and with Oreo you can't just do that easily but I have used SD Maid app in the Play Store and it works fairly well.
just needs to be done every time you have a major update or security patch update. Battery life will be impacted for a short. Of time until things work themselves out.
if you'd rather not use the app then you can go through the apps settings for each app and clear the cache for every app on the phone.
Please take a look at the post above and let us know if those steps help. Thanks.