FYI, I received this response from Purism support:
You can try disconnecting the battery from the MB and leave it like this over night, this has shown to help “reset” the battery. You can also try the reset button to see if it will help.
Guide to disconnect the battery: https://wiki.puri.sm/hw/L13/v3#disconnect-the-battery-connector
Apparently the reset button reboots the computer: Reset Button - what does it do?
1 day later:
Success! Disconnecting the battery overnight did the trick. Plugged it in this morning and was able to charge the battery to 100%. Will edit this post if I experience any more problems.