The 7 days in game time has long passed since I completed the tier 6 quests. In the attached screenshot you can see that the last tier 6 quest was made in game day 65 and we're now in game day 108:
as shown is this 2nd screenshot, only tier 4 are available now
this last screenshot shows that after completing all tier 5 quests, following the inavailability of tier 6 quests on day 65, tier 4 became the only option then.