Glad to see I'm not the only one having this issue. But for further details for DE: The counter reset is seemingly random; I've had it reset multiple times within the same mission (not always at the same number, from what I can tell, although the example below is around 130 kills), but consistently upon returning to my orbiter, even after successful completion in-mission. (Standing is not allotted upon completion either. I've had issues with the UI simply not updating in the past, but this is legitimately not clearing the task upon completion.) Progress also does not carry over between missions, as a result of resetting upon return to orbiter.
I grabbed ~15 minutes of gameplay (all in the same mission) showing the resets, in case it helps in diagnosing at all.
Successful 130/150 @ 2:10.
Reset to 1/150 @ 2:30.
Successful 130/150 @ 6:50.
Reset to 1/150 @ 7:50.
Successful 130/150 @ 11:30.
Successful 140/150 @ 11:40.
Successful 150/150 @ 12:00.
Reset on Orbiter @ 13:50.
For the record, while the video above is played solo (Friends Only, specifically), the problems exist while playing Public as well.
I'm guessing it's related to the doubled NW task, since I was able to complete the 25 scans daily task. (May or may not have reset upon return to orbiter before I completed it, though. Can't recall for certain, but it successfully cleared after I completed it within one mission.) My weeklies aren't giving me any issues either.