FP3 battery magically gains 10% without being charged

• /e/ version: 0.23-r-20220404175826-dev-FP3
• Device model(s): Fairphone 3
• Device rooted: no

The following happened to me yesterday and had already happened one time before. My FP3 was on with well below 20% of battery left (maybe around 14%), when I suddenly was unable to wake it up again from stand-by. After taking out the battery, putting it back in and plugging the USB-C cable in for charging, I was able to re-start the phone.

To my surprise, it displayed that the battery had magically gained more than 10% without being charged more than ca. 10 seconds.

Today, the same issue with the black display happened again – but this time, the battery magically lost ca. 30% within a couple of minutes. See my comment on #3531 (closed) for reference.

Here's a screenshot after the battery has magically gained more than 10%:

Here's a screenshot after the battery has lost ca. 30% within a couple of minutes: