This happens when your battery has gone through too many cycles. (drained and charged back up are 1 cycle)
After a couple of years, I eventually got to 1500 cycles and that was the point of no return.
You should start changing your battery settings to notify you when your battery level is 45% and critical notification at 35%
The higher the better though. And do use power saving whenever you're not gaming.
You can create a different power plan for general use/savings and another plan when you're gaming (High Performance).
Also, in Nvidia Control Panel, make sure global setting for power is "Optimal Power" or "Adaptive".
I'm unsure but I think for games, the setting changes to Maximum Performance, if not, you can do that manually for each game.
Some other tips include not putting your PC to sleep. Use hibernate instead. You can enable that option if it's not already.
Never let your laptop die. If you find it often dies, then increase the notification percentage.
If you're at 100% brightness, just go as low as you can for when you're not plugged in.
I regret not paying more attention to my battery. I also started seeing my battery not charge past a certain point.
Then it gradually got worse as the months passed and eventually I figured out a way to get it to 90% but it wasn't truly 90%.
It drained from 90% to 10% in probably minutes. Now my battery won't charge past 0% and will shut down if unplugged.
I suppose I could try getting a battery replacement but these Asus laptops make it very difficult and expensive.