Why is my iPod Touch 6 rebooting and not charging properly?

I recently bought a replacement battery (from ifixit) for a 6th gen iPod touch and I replaced the battery without much hassle (other than I broken screen but that was much easier to replace). I did all the soldering of the battery by myself and the joints looked good. When I put the screen back on and turned on the iPod, it seemed to work fine so I just let it sit for a while and waste charge so I could calibrate the battery. Now the iPod is doing a few abnormal things that I can't figure out the cause of.

The iPod seems to charge fine, getting all the way up to 100% charge at a normal speed. When it's unplugged, however, it reboots to the home screen and shows about 50% charge. The battery then seems to lose charge sporadically and quickly going from about 50% to around 20% in an hour or less. The iPod also occasionally reboots during button presses (home or sleep/wake button) or just when it feels like it.

The battery seems to go back to charging normally after it's plugged back in and it acts like it's completely fine.

At the time of writing this post, the iPod now only shows the Apple logo and goes to a black (but powered) screen and won't respond to any input. When it's unplugged, it won't power on, reboot, or in any way respond to button input.

This issue is probably the weirdest I've had in all of the repairs I've done. Does anyone have any input or ideas on the cause of this? Is this something stupid that I've done during the repair process or is this just a faulty battery?