Hi everyone, I noticed this problem occur for me for the first time when I turned off the setting to optimise the photos on my phone for storage space. My theory is that this "System Data" is actually the photos being downloaded off iCloud and it is attempting to save them back into the camera roll, but there isn't enough space, so it never actually registers as a "Photos" in the little gauge at the top... I'd recommend trying to turn that setting back on, if it is off at this point, and see if it helps.