I've noticed the same thing! When I first fill it, it vapes really well. When it's half empty, juice turns dark and tastes burnt.
ive noticed this too. i call that dark thick juice the sludge. i only get sludge if i refill the tank without cleaning it. i typically dump out the last 0.35ml, rinse out the tank parts, then replace the coil with a new, cleaned, or rebuilt head.
i never realized that the thinner parts of the jucie were gettign wicked and vaped first. i always thought that the juice got thicker and darker due to gettign heated up with every draw. i thought a few drips of heated juice would fall back down into the tank, and by the time the tank was nearly empty, youre left with jucie that has gotten heated but not vaped over and over.
but this only happens on certain juices. usually its the sweeter or darker ones.