Ok so can't really fix it unless you buy a new battery but what caused this is that you may have been on It while it's on a charger. Now I'm not saying you did this but battery's can die faster when used on a charger
I hope this helps, Thanks
battery dying depend (most of the time) on how you use it...
If you were watching bunch of High res. videos with speakers blasting or playing games for hrs at a time.. 6hrs seem quite reasonable.
But if it was just normal use of intermittent videos, web surfing, etc. etc. it certainly is an issue.
you could take it to UbreakIFix and have them take a look at the phone/battery.
you are correct in that it's not good for the battery... the tech has come a long way to smart-ly identify when a user is using the phone while plugged-in and will change the charging style to 'trickle'. BUT it's still bad for the battery. it's best to just use the battery then plug it in when you are ready to sleep. OR... I think I noticed a routine setting that will allow you to turn charging on/off at a specified time or command. You can try that and try to save your battery from the bad habit (next device at least).
I'm talking about Bixby routine by the way...