The week events do end before reset, you can see the start and end times on the ingame calendar.
Battle for Azeroth Dungeon event ran from 3rd July 9am Server time until 10th July 6am Server time.
Server time would be in CEST right now, that would have been about an hour before you wrote your post here this morning.
So this looks like the event ended earlier than you had expected it to, but it did end at the right time this morning and as the event was over the quest would no longer be available.