iPhone 6+ with iOS 8.3 - Cellular Data Being Used When Not Checked?

I have been having a problem with my cellular data usage on my iPhone 6 Plus since I updated to iOS 8.3. I had my iTunes and App Store settings set to automatically download Updates only (auto updates off for Music, Apps and Books) and "Use Cellular Date" switched off (i.e., my phone should use Wi-Fi only to update my apps but that does not appear to have been happening). Since updating to iOS 8.3, my phone has been using cellular data at times when I know it is connected to a good Wi-Fi network either at home or at work. My cellular service provider is Verizon.

I normally use about 3-5 GBs of data for my phone in a typical month. So far in April 2015 my phone has used over 14GB of data.

Has anyone had something similar happen? When I searched the forums there was an old thread (from 2012 or so) that indicated a similar prior problem was fixed when Verizon released a carrier update, but I do not see any carrier updates on my phone (version now is Verizon 19.0). Any ideas? Thanks!