[bluGill]

If you have a badly soldered (brazed, welded, or other way to join pipes) joint it will still hold pressure for a month or two. Then mechanical vibration will slowly degrade the joint and eventually metallic fatigue will cause a small break and then you get a leak.

Note that most of these leaks are caused by installation. In the factory they have things they can do for quality control to make such things not an issue, but the refrigerant needs to get from inside each house to the outside and that means custom install which makes quality control harder.

Worse, it is hard to find leaks. Often the pipes are buried in walls and so all the tech can do is put some sealer in and hope that fixes the issue. There is the ability to put some leak detector in the lines, but then you need to spend a lot of labor and still not guarantee.

I don't know what can be done about it, but I know it is a real problem.