Weird. They are quite literally my only source of juice, so I may be biased. I do quick steep their juice as I get them in the mail and then put the bottles I'm not using in a shoebox. (Put them in a ziplok bag and submerge them in warm water until it's cold, I do this 5 times) But I always use a bottle right out of the mail (after quick steeping).
I vape sweet stuff... candy, fruit etc. I get all of my juice in 80VG/20PG 3mg NIC with no flavor shots... and even with the lack of PG I get great flavor, consistently.
The flavor shots may have introduced a weird taste, I've never added shots to any juice from any vendor so I wouldn't know. If the KF4 is brand new, have you cleaned it before using it? It may have machine oil on it.