Extremely fast battery wear - asus rog zephyrus gx531 gv

I have been using my laptop - rog zephyrus gx531 gv for about 5 month and this problem just happened yesterday. When I first bought this laptop 5 month ago, I checkes the battery wear percentage and it was 2.9%. I mostly use my laptop pluged in because I play games a lot. I also turn on the balance battery mode inside the asus software so my battery only charged up to 80%, it never go higher than that. There wasnt any problem with battery wear, it stay at 2.9%, until yesterday, I came back from my holiday (3 days) and turned on my laptop. I checked the battery and it was 39.9%. I was really really really shocked and now i dont know how to deal with it.
I will be very thankful for any help from people.

Sometimes charging to 80% can mess with the battery monitoring system. To truly verify the wear, set the charger setting back to 100% and then run the laptop on battery low as possible while in OS. The OS will run till 5% or so. After that turn it off and turn it on to BIOS and leave it there till the laptop turns off. Plug the AC adapter in till battery charges to 100%, while the computer is off. Turn the laptop on after fully charges and verify the wear level.

Note: Li+ cell's life is based on date of manufacture. You had the laptop for 5 months but that doesn't mean the cell is only 5 months old.