Up to here linked page recommendations it is written: “Birch 8. When the red light turns off the battery is fully charged.” Please poweroff your Librem 5 Evergreen while on and (still) connected to its original power supply and wait until constant red LED light turns completely off. This should help to reestablish 100% of your Librem 5 battery capacity (up to 4.20V recommended max.). Charging rate should be around 5V/1.5A, even 5V/1.6A if your battery still healthy and not taking too long (such powered-off device charging will take several hours but might help with this quick discharging/drainage you described in your post, I hope so).
I needed to edit this post as same procedure already recommended in this thread: