Likely, the cracking you are experiencing is from liquid being trapped in a space with no room for expansion. Hence the cracks from the incredible amount of pressure you are creating in that space. dangerous explosive pressure.

Cold propane is not going to crack the brass nipples. Pressure will crack the nipples along the length usually staring in the center. its the weakest point and the easiest place to break.

The failure came from within. pressure caused this. I would highly reccommend you design more safety in your device before someone gets injured or killed.

This could be as simple as installing nitrogen bladder type accumulators in all of your liquid spaces, as well as using forged steel plumbing and cryogenic rated valves. It would also be helpful to use materials rated for what you are doing. A safe functional system that carries liquid propane is fiscally ten times more expensive to make safely than a well built vapor system.