A number of years ago the manufacturing of these items, along with many other similar components, moved to Mexico and in some cases the Pacific Rim countries. The quality of the brass castings went down badly. Cracking in the cast parts is not unusual since then, along with the quality of the machining of threads, etc. opening the door for leaks.
Since the fittings mentioned by the OP is installed on the high pressure side of the supply system it is exposed to pressures of 250 psi +/- and will leak much easier than the items on the low pressure side where pressures of about 6 ounces per square inch are normal.