On top of what Pixcel has said, make sure when you plug in the device to charge, the “Charging Assistant” notification is present. It shall say “Charging to 90%” as part of this.
If you don’t see the notification, assume it shall be charging to 100%. So it might be worth double checking your battery settings:
Settings > Battery > Battery health > Custom Charging Mode (is switched on). Then:
Settings > Battery > Charging Assistant > Display charging notification (is switched on).
If you face any further issues, please do submit your feedback, as suggested by Harry.
All the best with your fault finding 🤞🏼
