Why does Android OS data usage not count?
As far as I can tell, Android OS (sometimes called OS + others) is not actually a real application to say. Basing this on one data application I've used it is explained;


  It is a "catch all" pseudo-application accounting for the difference
  between global usage (displayed in the main screen) and the sum of all
  installed applications usages.



Initially it will be equal to global usage, but discrepancies can easily be noted in case when an application which previously had access to data connectivity is uninstalled.
On the other hand, the value can stay as a very small value (few MBs) perhaps if the applications that occasionally use data are not affected (remain intact).
The other reason, I can think of is, data used in some minor updates (to Android itself), connectivity checks etc  can be assigned to Android OS in data usage counter such that there is always a reading for such in data counters.


I don't think this has something to do with Lineage OS or Nougat but just a conventional way for counting data usage accross apps (regardless they were uninstalled in that particular usage period)