A few clarifications. My usual practice is to connect the Kobo to Calibre almost every day, which means that it is always charged between 90% and 100%.
This is probably not the best for the battery, and the BMS, as I know it on other types of devices, needs a complete discharge followed by a complete recharge from time to time for a good cell balance.
So I stopped connecting the Kobo to the PC some time ago, waiting for the battery to drain as much as possible, trying to prevent it from draining completely, which is not good either.
The Kobo is currently in charge. I'll see if that solves the problem or not.
(This English version of my message is from an automatic translator
www.DeepL.com/Translator)