Phone doesn't charge after ifixit battery replacement.

So I ordered a battery and screen adhesive from ifixit so I can replace my iPhone 12 Pro battery.

At first boot with the new battery, phone reported black screen with "battery needs charging" icon. Alright, plugged it in and it started no problem. I read that sometimes the phone needs charging juice to turn on.

It turned on and reported 60% something percent charge. Alright, following the battery calibration guide on ifixit website and on the battery's box, I left the phone to charge until it reaches 100% and leave it charging for at least 2 more hours.

Done so and came back after 6 hours to phone stuck at 94% charge. Battery optimization was enabled at the time. There was no mention of this feature in the guide. I disabled battery optimization and put the phone to charge again. Came back after 2 hours and phone was still stuck at 94% charge?

I force restarted the phone while it was charging and it showed 98% charge. Cool. I'll leave it to charge to 100% then. Except... it didn't charge...

It stayed at 98%. I thought I would have to drain it completely and charge it back up so I started using it until it would shut down, no problem. Came back home and phone went to 0%. Cool. I left it to charge overnight and in the morning I woke up with 70% battery. When I unplugged it, the % started to go down about 1% every few minutes. Strange... Plugging the phone back in told me it was charging but the % was still decreasing. Force restarted again, it showed 94% battery and it started dropping fast again.

Now I'm in the situation where I think the ifixit battery is low on charge and the phone refuses to charge it. What else can I try? Is my ifixit battery DOA and I need to get a new one (and new screen adhesive)?