Why won't my joycons fully charge?

I recently bought a new battery replacement for my left-joycon. I switch the battery and put in the new one. At first I notice that it wouldn't turn on (I thought I had to charge it first, so that is what I did.) I connected my left-joycon to the main system and let it charge for two hours. Everything charged (main console and right-joycon were at full charge), except for the left joycon. Then I notice, I was able to use the left-joycon while it was attached to the main system, but as soon as I disconnected from the main system, the left-joycon wouldn't turn on. The whole time that the left-joycon was connected to the main system, I could see that the battery indicator was red, while the main sytem and right-joycon were at full battery. Is this a battery issue? Defected battery? Or did I do something wrong? I also got the battery replacement today (08/21/2020).