Did you experience something similar? I did a factory reset on my FP2 and it keep some files readable on the device. I noticed that in the Galerie. Around 9 images were still on the device after the reset, among those images that were send to me using Threema and Screenshots. I do not have an SD Card in the device.
Can someone run
mount
in an adb shell on a FP2 with lineage os?