The battery is used to “top up” certain power draws using the battery and will not start recharging again until it drops a certain percentage. For me, this usually involved the dGPU. Over time, this naturally drains the battery. Under light load though, I would’ve anticipated this going from 100% → 95% at most though, do you have a battery limit set to 85%? The battery charging logic on the Framework is quite different, at least in how it presents itself to me, from other laptop’s I’ve had. If you were gaming, you’d also likely be running into an issue where your performance suddenly tanks when the battery hits a certain threshold.
There’s a Framework 13 thread that discusses battery flipping in more detail.
It does seem odd if it’s happening under a truly light load though. I don’t recall that happening to me, but I also haven’t looked.