2019 Kia Forte has a dead battery

I tried to start my car and got nothing, the battery was reading 11.4 volts, so I waited until some people woke up to show them what it was doing, when I tried again. This is when I noticed it started on just 8.5 volts instead of what I thought was the minimum of 9.6 so I charged it for a few hours and it wouldn’t go past 80% even after turning it off and trying again on both a trickle charge and a 15a

I took it off the charger and ran a battery health, cranking and ripple test and they all tested good, for a few hours anyway then it was dead again, the alternator was good on DC, but the AC setting from the battery was spiking all over the place so I just figured my multimeter didn’t read rms, there wasn’t a substantial parasitic draw, I’m always driving long and short distances, there wasnt any sulfur on the terminals and Its a 2019. I took the battery to Walmart and they said we don’t carry that battery to my surprise, I went to Autozone and they said, they haven’t made that battery available to us because it’s a new type of battery, at this point I’m in ahh.

I called the dealer and it’s going to be roughly $300 for a battery for a brand new car, why would a "brand new battery" die so quick and why cant a kia forte use any type of battery with 640 CCA 100 Rc, is it possible that they designed a bad battery, If it was the alternator spitting out too much AC to the battery then I would of imaged the electronic battery health check tool would of caught it on the charging test.