So today, I checked the hive and 7/10 of the frames were drawn out. I saw a fair amount of brood, and then pollen above it, then nectar/syrup. The hive is only 2 weeks old, was it too soon to add another super? since in about a week or two, about a 1/3 will die off?
I also put the top-feeder back on and loaded it up with syrup again.
Is it normal for the bees to cap the syrup?
The bees are in charge of telling you when, how soon, how often, they need supering. Your job is to learn the signs and respond. According to the 70/30 rule, when 70% of the frames are covered with bees it is time to swarm because with the hatching of the new bees means they will fill up that remaining 30% in a matter of days. So if you have 7 out of 10 frames full of bees it is time to super it 2 weeks old or not.
Some people recommend feeding the bees until they have all the frames in 2 boxes completely drawn out. I don't I feed my bees 2 gallons of syrup at the rate of 1 gallon, wait a week, and feed another gallon so that the bees do not impede the queens egg laying by backfilling what should be the brood chamber with syrup. It is the new bees that builds the population of the hive so it is simply too easy to overfeed the bees which can cause even a new package or swarm to swarm due to congestion. Congestion can be from too many bees for the available space or too much honey for the queen to lay eggs, either will can cause a swarm.
Bees will cap syrup or nectar when it is cured (moisture removed), to them syrup and nectar are the same thing.