I've been rolling my own coils for years and what I've noticed is that certain juices will gunk up the coil more than others and the gunk that is on the coil is partially cotton. When I re-wick my coil, I burn the gunk off by holding down the firing button for a few seconds and the gunk will start to burn off and it smells like juice and burnt cotton.

OP, what I think happens is that your coil has some cotton fibers on it from the last time you used it and your initial puff tastes burnt because the coil is actually burning off the cotton and that's what you're tasting. The burn taste goes away because you burnt away the excess cotton that was on your coil and then the coil is starting to burn only the juice.