Interesting trend I observed in more recent data logging:
In a test sketch, I have a Photon that is never going to sleep (because there's a bug with it connecting to wifi after, grr) and every 5 minutes (millis + 300000) it publishes an event that IFTTT dumps into a google spreadsheet.
Going by the spreadsheet, the battery reported the 255.9960375 soc until it flatlined and died. I plugged it into a power source, and the first couple readings were actually different, believable figures, as it began charging! And with each reading the voltage rose, until it hit the 5.1187, at which point the SOC went haywire again. So, if the voltage is ramping up, it is fine, but at typical it borks it?
More as more data arrives...