I've had this two times on two consecutive days now, with similar usage situations (short phonecalls, a couple of messages and light net surfing prior to incident): the battery has been discharged from full-ish to just under the 70%- mark through normal use, when suddenly the charge drops to 4 % (yesterday 65->4, today 69->4). The first time I just did a reboot to check if the battery level returned to normal (it didn't), and plugged the phone to the charger and left it there. On todays sudden drain at first I did the same, but then decided to unplug the charger and do a reboot. The charge level rose immediately from 25% to 76% after it. I also noticed that the charging seemed quite quick before the reboot, but it dropped to a normal rate after it.
-I checked the Notifications just before both incidences; Twitter, Gmail and FB sync on
-No Developer mode activated, no OpenRepos
-Apps running ATT: Meecast, Messages, Lighthouse, Phone, Jolla Store
-This might be irrelevant, but during these two days in question I had a couple of warnings about the unit overheating, and being in need of a reboot. The phone temperature seemed normal to me, and Lighthouse clocked CPU temps between 38 and 41 Celsius...
So, does this seem like a software issue, or a problem with the battery?
Edit: added tag "temperature"
Update 12.1. 2015: Closed this, as the issue has apparently been fixed, or was just minor glitch. Still no actual answer to the question, but judging by the lack of votes and comments hasn't happened that often lately... Also retagged the question (added "bug").
Update 13.1. 2015: Reopened the question, as it seems that others have had this issue quite recently after all. Sorry for going back and forth with this :)