It's late in the iOS 12 cycle, but with iOS 12.3.1 I see a resurgence of an old bug from iOS 7 days. Specifically, iOS allows applications such as OmniFocus and Overcast to use mobile data, although I have not checked the Settings> Mobile. I have had this off for many years for some apps in an effort to reduce my data usage. Especially, OmniFocus can use more data than all of my other apps combined, if I leave, and that's unnecessary because I've already synced the latest data via Wi-Fi before I leave my Mac.
Of course, the reason I noticed this was that my cell schedule ran out of data two weeks early and that never ran out of data. The first thing I did was to check Settings, and all the data-displaying apps were at the top of the list, despite being unchecked. (I also confirmed that the Wi-Fi Assist was turned off and the problem persisted after restart.)
Omni's support confirmed that the app does not have control over it ̵
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