Weystones - Map Tiers - Map issue

I constantly have to re-farm from Tier 3 to Tier 6 Weystones in a very annoying way...

Every time I get a Tier 6 map I get unlucky and only see Tier 2-3 maps drop within the instance. I then have to grind from Tier 3-4 to 6 again. If we were to "push" for higher tiers, it would be a huge challenge in the current state. I am upgrading the maps but it doesn't help.