I bought a used kayfun a while back out of FB. It was my first and last FB buy other than coops. It had something awful and strong in it and then to boot it had a stale juice smell. I broke it down into bits, soaked in isoprop, boiling water, found every nook n cranny and still YUK. I ended up putting a strong menthol into it and eventually the menthol prevailed.
I have noticed too but will try again with another batch is that the Hangsen lemon was great first mixed and a few days and now after sitting a week+, a drop on the back of the hand tastes good but in the atty its flat, and very little taste.
I scale out my base then tap hot water I warm it up a bit before adding my flavor and nic. The warmth helps incorporate the flavors better but just barely warm. I don't believe in heat steeping in any way since it wrecked a few juices I had in my earlier vaping days. Having a DIY magnetic mixer is great and does a wondrous job.