I noticed that my filter system was leaking too. I found this out when I was back there to install my own 2-stage water filtration system. I didn't know the system was leaking at first. I thought the leak was at the pump right below the filter. So I was chasing the wrong thing. In the end, I removed the Heartland supplied filter system and used a 1/2" x 1/2" plastic union to connect the in/out lines together.
Later, I noticed the connection was still dripping a bit. My guess is that either one of the connectors is defective or one of the PEX crimp rings is not crimped all the way. I plan to tear into it this coming week.
I suggest you dry the filter housing and both fitting really well, then just wait and watch to see where the water beads up at. If you can get it real dry, one aid is to use toilet paper as a marker. Wrap the fittings with toilet paper and you'll know soon, when end the leak is at. Then fix from there.
It's a long shot, but perhaps the rubber o-ring/gasket wasn't installed in the top of the filter housing and the housing was just cranked on as tight as possible. The housings (canisters) can be a real bear to crack loose, especially without a filter wrench.