Well, Ive been bitten by whatever this bug is. The example just today.
Get a text this monring saying I have used 100% of my data. Fido site (and app) says 1.99 GB remaining...now this is based on 12 hours ago but keep in mind that over that 12 hour period I was at home and for 7 or so hours asleep. My iPhone says I have only used 446 MB since November 10th (on the 9th I got hit with it say 75% used and to be safe added 1GB and decided to reset the usage stats).
So, have used only 446 MB in 7 days (almost nothing uses cellular data, photos don't upload unless I'm on wifi, etc.) but Fido is telling my that I've blown through over 2GB of data...somehting is not right here. Unless there is somehting seriously wrong in iOS 11 in terms of a data leak to cellular when on WiFi (which I doubt), I'm feeling the issue must be on Fido's usage tracking.
Before iOS 11 I averaged at most 5GB a month (got 6 to use) but since the update, the usgage being stated by Fido is completely out of control compared to what my phone is saying I've used.
To be compeltely safe I've litterly turned off data completely...if this keeps croping up, I think it's time to start thinking about a move since I don't need this kind of headache and worry
Did you get a chance to check out the usage summary for your Apps? You can usually do this by going through Settings > Cellular.
Can you take a look and let us know if anything seems unusual in any of your Apps usage?